More documentation

docs/features/authentication.md

@@ -1,6 +1,7 @@
 # Authentication
-Performed via [synapse with REST auth module](https://github.com/kamax-io/matrix-synapse-rest-auth/blob/master/README.md)  
-Point the `endpoint` to mxisd internal IP on port 8090
+Authentication is an enchanced Identity feature of mxisd to ensure coherent and centralized identity management.
+
+It allows to use Identity stores configured in mxisd to authenticate users on your Homeserver.
 
 ## Overview
 ```
@@ -23,6 +24,26 @@ Point the `endpoint` to mxisd internal IP on port 8090
               |   user profiles          |    If valid credentials and supported by backend
               +--------------------------+
 ```
+Performed on [synapse with REST auth module](https://github.com/kamax-io/matrix-synapse-rest-auth/blob/master/README.md)
+
+## Getting started
+### Synapse
+You will need:
+- Configure and enable at least one [Identity store](../backends/)
+- Install the [REST auth module](https://github.com/kamax-io/matrix-synapse-rest-auth)
+
+Once installed, edit your synapse configuration as described for the auth module:
+- Set `endpoint` to `http://mxisdAddress:8090` - Replace `mxisdAddress` to an internal IP/Hostname.
+- If you want to avoid [known issues](https://github.com/matrix-org/matrix-doc/issues/586) with lower/upper case
+usernames, set `enforceLowercase` in the REST config to `true`.
+
+**IMPORTANT**: if this is a new installation, it is highly recommended to enforce lowercase, as it is not possible to
+workaround the bug at a later date and will cause issues with invites, searches, authentication.
+
+Restart synapse and login on the Homeserver using credentials present in your backend.  
 
 ## Profile auto-fill
-To be documented
+Auto-filling user profile depends on two conditions:
+- The REST auth module is configured for it, which is the case by default
+- Your Identity store is configured to provide profile data. See your Identity store [documentation](../backends/) on
+how to enable the feature.