Update CHANGES and UPGRADE

CHANGES.rst

+Changes in synapse vx.x.x (x-x-x)
+=================================
+
+* Add support for registration fallback. This is a page hosted on the server
+  which allows a user to register for an account, regardless of what client
+  they are using (e.g. mobile devices).
+
 Changes in synapse v0.7.1 (2015-02-19)
 ======================================
 

UPGRADE.rst

+Upgrading to vx.xx
+==================
+
+Servers which use captchas will need to add their public key to::
+
+  static/client/register/register_config.js
+
+    window.matrixRegistrationConfig = {
+        recaptcha_public_key: "YOUR_PUBLIC_KEY"
+    };
+
+This is required in order to support registration fallback (typically used on
+mobile devices).
+
 Upgrading to v0.7.0
 ===================