Start some documentation on password providers

Document the existing interface, before I start adding new stuff.

docs/password_auth_providers.rst

+Password auth provider modules
+==============================
+
+Password auth providers offer a way for server administrators to integrate
+their Synapse installation with an existing authentication system.
+
+A password auth provider is a Python class which is dynamically loaded into
+Synapse, and provides a number of methods by which it can integrate with the
+authentication system.
+
+This document serves as a reference for those looking to implement their own
+password auth providers.
+
+Required methods
+----------------
+
+Password auth provider classes must provide the following methods:
+
+*class* ``SomeProvider.parse_config``\(*config*)
+
+    This method is passed the ``config`` object for this module from the
+    homeserver configuration file.
+
+    It should perform any appropriate sanity checks on the provided
+    configuration, and return an object which is then passed into ``__init__``.
+
+*class* ``SomeProvider``\(*config*, *account_handler*)
+
+    The constructor is passed the config object returned by ``parse_config``,
+    and a ``synapse.handlers.auth._AccountHandler`` object which allows the
+    password provider to check if accounts exist and/or create new ones.
+
+``someprovider.check_password``\(*user_id*, *password*)
+
+    This is the method that actually does the work. It is passed a qualified
+    ``@localpart:domain`` user id, and the password provided by the user.
+
+    The method should return a Twisted ``Deferred`` object, which resolves to
+    ``True`` if authentication is successful, and ``False`` if not.