Update README.md (awslabs#2040)

* Update README.md

Add notification on the master branch which is now replaced by the main branch.

* Update README.md

Use new markdown for Warning.

README.md

@@ -1,3 +1,5 @@
+>[!WARNING]
+>This branch is deprecated and no longer receives updates. Please use the [latest release](https://github.com/awslabs/amazon-kinesis-video-streams-webrtc-sdk-c/releases/latest) or the [main branch](https://github.com/awslabs/amazon-kinesis-video-streams-webrtc-sdk-c/tree/main).
 <h1 align="center">
   Amazon Kinesis Video Streams C WebRTC SDK
   <br>