Add authentication docs.

docs/authentication.md

@@ -0,0 +1,11 @@
+# Authentication
+Once the connection is setup, the client can authenticate with the AP. For this, it sends an
+`ClientResponseEncrypted` message, using packet type `0xab`.
+
+A few different authentication methods are available, athough only one has really been tried, the
+traditional user / password based authentication.
+
+The AP will then reply with either a `APWelcome` message using packet type `0xac` if authentication
+is successful, or an `APLoginFailed` with packet type `0xad` otherwise.
+
+TODO: investigate other authentication methods (Zeroconf, facebook, stored credentials)