Updated UPDATING.md doc, deprecated sync API

UPDATING.md

@@ -72,6 +72,7 @@ These include:
 
  - Deprecation of support for Python versions 3.4, 3.5 and 3.6
  - New, asynchronous API
+ - Deprecated synchronous API
 
 ### Deprecation of Python 3.4, 3.5 and 3.6
 
@@ -85,6 +86,23 @@ To see which versions of Python we test the SDK against, please look at our
 The 1.2.0 version introduces a breaking change, which changes the way of interacting with the SDK from synchronous to asynchronous, using [the `asyncio` foundational library](https://docs.python.org/3.7/library/asyncio.html) to provide support for `async`/`await` syntax.
 Because of this breaking change, every call that interacts with the Ably REST API must be refactored to this asynchronous way.
 
+Important Update - 
+- If you want to keep using old synchronous style API, make sure to set `sync_enabled` property of `AblyRest` client.
+- This is applicable for all new Ably REST async APIs.
+
+```python
+from ably import AblyRest
+
+def main():
+    ably = AblyRest('api:key')
+    ably.sync_enabled = True
+    channel = ably.channels.get("channel_name")
+    channel.publish('event', 'message')
+
+if __name__ == "__main__":
+    main()
+```
+
 #### Publishing Messages
 
 This old style, synchronous example: