Notice about change to main as default branch

Author: t0shiii

README.md

@@ -2,6 +2,8 @@
 
 [![Code Climate](https://codeclimate.com/github/aws/aws-codedeploy-agent.png)](https://codeclimate.com/github/aws/aws-codedeploy-agent) [![Build Status](https://travis-ci.org/aws/aws-codedeploy-agent.png?branch=master)](https://travis-ci.org/aws/aws-codedeploy-agent) [![Coverage Status](https://coveralls.io/repos/aws/aws-codedeploy-agent/badge.svg?branch=master&service=github)](https://coveralls.io/r/aws/aws-codedeploy-agent?branch=master)
 
+# Notice: This branch is no longer the default.  Please use `main` branch.
+
 ## Latest Release: 1.4.0
 [Release Notes](https://docs.aws.amazon.com/codedeploy/latest/userguide/codedeploy-agent.html#codedeploy-agent-version-history)