Add user documentation for JSON Home

Keystone has supported JSON Home documents since Juno, but we never
had any user-facing documentation for the API. This commit adds a
section to the user guide that describes what JSON Home is and adds
an example of how users can get it.

Change-Id: Ib0793f6af4f65e5549ba0543b87d20f3f1a8a62d

doc/source/user/index.rst

@@ -26,4 +26,5 @@ An end user can find the specific API documentation here, `OpenStack's Identity
     :maxdepth: 1
 
     application_credentials.rst
+    json_home.rst
     ../api_curl_examples.rst

doc/source/user/json_home.rst

@@ -0,0 +1,84 @@
+..
+      All Rights Reserved.
+
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+============================
+API Discovery with JSON Home
+============================
+
+What is JSON Home?
+==================
+
+JSON Home describes a method of API discovery for non-browser HTTP clients. The
+`draft`_ is still in review, but keystone supplies an implementation accessible
+to end-users. The result of calling keystone's JSON Home API is a JSON document
+that informs the user about API endpoints, where to find them, and even
+information about the API's status (e.g. experimental, supported, deprecated).
+More information keystone's implementation of JSON Home can be found in the
+`specification`_.
+
+.. _`draft`: https://mnot.github.io/I-D/json-home/
+.. _`specification`: http://specs.openstack.org/openstack/keystone-specs/specs/keystone/juno/json-home.html
+
+Requesting JSON Home Documents
+==============================
+
+Requesting keystone's JSON Home document is easy. The API does not require a
+token, but future implementations might expand in it's protection with token
+validation and enforcement. To get a JSON Home document, just query a keystone
+endpoint with ``application/json-home`` specified in the ``Accept`` header:
+
+.. code-block:: bash
+
+   curl -X GET -H "Accept: application/json-home" http://example.com/identity/
+
+The result will be a JSON document containing a list of ``resources``:
+
+.. code-block:: console
+
+   {
+       "resources": [
+           "https://docs.openstack.org/api/openstack-identity/3/ext/OS-TRUST/1.0/rel/trusts": {
+               "href": "/v3/OS-TRUST/trusts"
+           },
+           "https://docs.openstack.org/api/openstack-identity/3/ext/s3tokens/1.0/rel/s3tokens": {
+               "href": "/v3/s3tokens"
+           },
+           "https://docs.openstack.org/api/openstack-identity/3/rel/application_credential": {
+               "href-template": "/v3/users/{user_id}/application_credentials/{application_credential_id}",
+               "href-vars": {
+                   "application_credential_id": "https://docs.openstack.org/api/openstack-identity/3/param/application_credential_id",
+                   "user_id": "https://docs.openstack.org/api/openstack-identity/3/param/user_id"
+               }
+           },
+           "https://docs.openstack.org/api/openstack-identity/3/rel/auth_catalog": {
+               "href": "/v3/auth/catalog"
+           },
+           "https://docs.openstack.org/api/openstack-identity/3/rel/auth_domains": {
+               "href": "/v3/auth/domains"
+           },
+           "https://docs.openstack.org/api/openstack-identity/3/rel/auth_projects": {
+               "href": "/v3/auth/projects"
+           },
+           "https://docs.openstack.org/api/openstack-identity/3/rel/auth_system": {
+               "href": "/v3/auth/system"
+           },
+           ...
+       ]
+   }
+
+The list of resources can then be parsed based on the relationship key for a
+dictionary of data about that endpoint. This includes a path where users can
+find interact with the endpoint for that specific resources. API status
+information will also be present.