Add documentation for JWT login type and improve sample config. (#7776)

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+# JWT Login Type
+
+Synapse comes with a non-standard login type to support
+[JSON Web Tokens](https://en.wikipedia.org/wiki/JSON_Web_Token). In general the
+documentation for
+[the login endpoint](https://matrix.org/docs/spec/client_server/r0.6.1#login)
+is still valid (and the mechanism works similarly to the
+[token based login](https://matrix.org/docs/spec/client_server/r0.6.1#token-based)).
+
+To log in using a JSON Web Token, clients should submit a `/login` request as
+follows:
+
+```json
+{
+  "type": "org.matrix.login.jwt",
+  "token": "<jwt>"
+}
+```
+
+Note that the login type of `m.login.jwt` is supported, but is deprecated. This
+will be removed in a future version of Synapse.
+
+The `jwt` should encode the local part of the user ID as the standard `sub`
+claim. In the case that the token is not valid, the homeserver must respond with
+`401 Unauthorized` and an error code of `M_UNAUTHORIZED`.
+
+(Note that this differs from the token based logins which return a
+`403 Forbidden` and an error code of `M_FORBIDDEN` if an error occurs.)
+
+As with other login types, there are additional fields (e.g. `device_id` and
+`initial_device_display_name`) which can be included in the above request.
+
+## Preparing Synapse
+
+The JSON Web Token integration in Synapse uses the
+[`PyJWT`](https://pypi.org/project/pyjwt/) library, which must be installed
+as follows:
+
+ * The relevant libraries are included in the Docker images and Debian packages
+   provided by `matrix.org` so no further action is needed.
+
+ * If you installed Synapse into a virtualenv, run `/path/to/env/bin/pip
+   install synapse[pyjwt]` to install the necessary dependencies.
+
+ * For other installation mechanisms, see the documentation provided by the
+   maintainer.
+
+To enable the JSON web token integration, you should then add an `jwt_config` section
+to your configuration file (or uncomment the `enabled: true` line in the
+existing section). See [sample_config.yaml](./sample_config.yaml) for some
+sample settings.
+
+## How to test JWT as a developer
+
+Although JSON Web Tokens are typically generated from an external server, the
+examples below use [PyJWT](https://pyjwt.readthedocs.io/en/latest/) directly.
+
+1.  Configure Synapse with JWT logins:
+
+    ```yaml
+    jwt_config:
+        enabled: true
+        secret: "my-secret-token"
+        algorithm: "HS256"
+    ```
+2.  Generate a JSON web token:
+
+    ```bash
+    $ pyjwt --key=my-secret-token --alg=HS256 encode sub=test-user
+    eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ0ZXN0LXVzZXIifQ.Ag71GT8v01UO3w80aqRPTeuVPBIBZkYhNTJJ-_-zQIc
+    ```
+3.  Query for the login types and ensure `org.matrix.login.jwt` is there:
+
+    ```bash
+    curl http://localhost:8080/_matrix/client/r0/login
+    ```
+4.  Login used the generated JSON web token from above:
+
+    ```bash
+    $ curl http://localhost:8082/_matrix/client/r0/login -X POST \
+        --data '{"type":"org.matrix.login.jwt","token":"eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ0ZXN0LXVzZXIifQ.Ag71GT8v01UO3w80aqRPTeuVPBIBZkYhNTJJ-_-zQIc"}'
+    {
+        "access_token": "<access token>",
+        "device_id": "ACBDEFGHI",
+        "home_server": "localhost:8080",
+        "user_id": "@test-user:localhost:8480"
+    }
+    ```
+
+You should now be able to use the returned access token to query the client API.