Add docs to Tower configuration.

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+Tower configuration gives tower users the ability to adjust multiple runtime parameters of Tower, thus take fine-grained control over Tower run.
+
+## Usage manual
+
+#### To use
+The REST endpoint for CRUD operations against Tower configurations is `/api/<version #>/settings/`. GETing to that endpoint will return a list of available Tower configuration categories and their urls, such as `"system": "/api/<version #>/settings/system/"`. The URL given to each category is the endpoint for CRUD operations against individual settings under that category.
+
+Here is a typical Tower configuration category GET response.
+```
+GET /api/v2/settings/github-team/
+HTTP 200 OK
+Allow: GET, PUT, PATCH, DELETE, HEAD, OPTIONS
+Content-Type: application/json
+Vary: Accept
+X-API-Node: tower
+X-API-Query-Count: 6
+X-API-Query-Time: 0.004s
+X-API-Time: 0.026s
+
+{
+    "SOCIAL_AUTH_GITHUB_TEAM_CALLBACK_URL": "https://towerhost/sso/complete/github-team/",
+    "SOCIAL_AUTH_GITHUB_TEAM_KEY": "",
+    "SOCIAL_AUTH_GITHUB_TEAM_SECRET": "",
+    "SOCIAL_AUTH_GITHUB_TEAM_ID": "",
+    "SOCIAL_AUTH_GITHUB_TEAM_ORGANIZATION_MAP": null,
+    "SOCIAL_AUTH_GITHUB_TEAM_TEAM_MAP": null
+}
+```
+
+The returned body is a JSON of key-value pairs, where the key is the name of Tower configuration setting, and the value is the value of that setting. To update the settings, simply update setting values and PUT/PATCH to the same endpoint.
+
+#### To develop
+Each Django app in tower should have a `conf.py` file where related settings get registered. Below is the general format for `conf.py`:
+
+```python
+# Other dependencies
+# ...
+
+# Django
+from django.utils.translation import ugettext_lazy as _
+
+# Tower
+from awx.conf import fields, register
+
+# Other dependencies
+# ...
+
+register(
+  '<setting name>',
+  ...
+)
+# Other setting registries
+```
+
+`register` is the endpoint API for registering individual tower configurations:
+```
+register(
+    setting,
+    field_class=None,
+    **field_related_kwargs,
+    category_slug=None,
+    category=None,
+    depends_on=None,
+    placeholder=rest_framework.fields.empty,
+    feature_required=rest_framework.fields.empty,
+    encrypted=False,
+    defined_in_file=False,
+)
+```
+Here is the details of each argument:
+
+| Argument Name               | Argument Value Type                                                  | Description                                                                                                                                                                   |
+|--------------------------|-------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `setting`                | `str`                                                             | Name of the setting. Usually all-capital connected by underscores like `'FOO_BAR'`                                                                                            |
+| `field_class`            | a subclass of DRF serializer field available in `awx.conf.fields` | The class wrapping around value of the configuration, responsible for retrieving, setting, validating and storing configuration values.                                       |
+| `**field_related_kwargs` | **kwargs                                                          | Key-worded arguments needed to initialize an instance of `field_class`.                                                                                                       |
+| `category_slug`          | `str`                                                             | The actual identifier used for finding individual setting categories.                                                                                                         |
+| `category`               | transformable string, like `_('foobar')`                          | The human-readable form of `category_slug`, mainly for display.                                                                                                               |
+| `depends_on`             | `list` of `str`s                                                  | A list of setting names this setting depends on. A setting this setting depends on is another tower configuration setting whose changes may affect the value of this setting. |
+| `placeholder`            | transformable string, like `_('foobar')`                          | A human-readable string displaying a typical value for the setting, mainly used by UI                                                                                         |
+| `feature_required`       | `str`                                                             | Indicator of which feature this setting belongs, a user whose license does not allow a feature cannot access its related settings.                                            |
+| `encrypted`              | `boolean`                                                         | Flag determining whether the setting value should be encrypted                                                                                                                |
+| `defined_in_file`        | `boolean`                                                         | Flag determining whether a value has been manually set in settings file.                                                                                                      |
+
+During Tower bootstrapping, All settings registered in `conf.py` modules of Tower Django apps will be loaded (registered). The set of Tower configuration settings will form a new top-level of `django.conf.settings` object. Later all Tower configuration settings will be available as attributes of it, just like normal Django settings. Note Tower configuration settings take higher priority over normal settings, meaning if a setting `FOOBAR` is both defined in a settings file and registered in a `conf.py`, the registered attribute will be used over the defined attribute every time.
+
+Starting from 3.2, Tower configuration supports category-specific validation functions. They should also be defined under `conf.py` in the form
+```python
+def custom_validate(serializer, attrs):
+    '''
+    Method details
+    '''
+```
+Where argument `serializer` refers to the underlying `SettingSingletonSerializer` object, and `attrs` refers to a dictionary of input items.
+
+Then at the end of `conf.py`, register defined custom validation methods to different configuration categories (`category_slug`) using `awx.conf.register_validate`:
+```python
+# conf.py
+...
+from awx.conf import register_validate
+...
+def validate_a(serializer, attrs):
+...
+def validate_b(serializer, attrs):
+...
+# At the end of conf.py
+register_validate("category_a", validate_a)
+register_validate("category_b", validate_b)
+...
+```