chore(versioning): add versioning explanation for master branch and symfony_3_4 branch

Author: ArturMoczulski

README.md

@@ -5,7 +5,11 @@
 
 Rollbar full-stack error tracking for Symfony.
 
-Supported Symfony versions: 5+.
+Supported Symfony versions: 5+
+
+Support for Symfony versions 5+ is maintained in branch `master` and tags `v4+`.
+
+Support for Symfony versions 3.* and 4.* is maintained in branch `symfony_3_4` and tags in the `v3.*` series.
 
 ## Setup Instructions
 1. [Sign up for a Rollbar account](https://rollbar.com/signup)