Add details of configuration options for synapse module

Author: hughns

synapse/README.rst

@@ -5,9 +5,9 @@ This is an implementation of `MSC3886
 <https://github.com/matrix-org/matrix-spec-proposals/pull/3886>`_ for `Synapse
 <https://github.com/matrix-org/synapse>`_.
 
-=====
+-----
 Usage
-=====
+-----
 
 1. Install the module to make it available to your Synapse environment::
 
@@ -28,3 +28,24 @@ Usage
 4. Run Synapse with the asyncio reactor enabled::
 
     SYNAPSE_ASYNC_IO_REACTOR=1 python -m synapse.app.homeserver
+
+---------------------
+Configuration options
+---------------------
+
+Apart from the `prefix` the following config options are available:
+
+- `ttl`: The time-to-live of the rendezvous session in seconds. Defaults to 60.
+- `max_bytes`: The maximum number of bytes that can be sent in a single request. Defaults to 4096.
+
+An example configuration setting these and a custom prefix would like::
+
+    modules:
+      - module: matrix_http_rendezvous_synapse.SynapseRendezvousModule
+        config:
+          prefix: /rendezvous
+          ttl: 15                   # seconds
+          max_bytes: 10240          # 10 KiB
+
+    experimental_features:
+      msc3886_endpoint: /rendezvous # this should match above