add API documentation

Signed-off-by: Joseph Weston <joseph@weston.cloud>

docs/admin_api/version_api.rst

@@ -0,0 +1,22 @@
+Version API
+===========
+
+This API returns the running Synapse version and the Python version
+on which Synapse is being run. This is useful when a Synapse instance
+is behind a proxy that does not forward the 'Server' header (which also
+contains Synapse version information).
+
+The api is::
+
+    GET /_matrix/client/r0/admin/server_version
+
+including an ``access_token`` of a server admin.
+
+It returns a JSON body like the following:
+
+.. code:: json
+
+    {
+        "server_version": "0.99.2rc1 (b=develop, abcdef123)",
+        "python_version": "3.6.8"
+    }