Backwards incompatibility warning for ThirdPartyEventRules in UPGRADE.rst

UPGRADE.rst

@@ -127,6 +127,17 @@ request to
 with the query parameters from the original link, presented as a URL-encoded form. See the file
 itself for more details.
 
+ThirdPartyEventRules breaking changes
+-------------------------------------
+
+This release introduces a backwards-incompatible change to modules making use of
+`ThirdPartyEventRules` in Synapse.
+
+The `http_client` argument is no longer passed to modules as they are initialised. Instead,
+modules are expected to make use of the `http_client` property on the `ModuleApi` class.
+Modules are now passed a `module_api` argument during initialisation, which is an instance of
+`ModuleApi`.
+
 Upgrading to v1.18.0
 ====================