Document new 'v2' branch meant for unstable development.

This patch documents the newly added v2 branch, which will
be used to stage, test, and receive feedback on upcoming
features, most of which will be breaking changes which can't
be directly applied to master.

README.md

@@ -45,6 +45,18 @@ sudo make install
 Now you have google/benchmark installed in your machine 
 Note: Don't forget to link to pthread library while building
 
+## Stable and Experimental Library Versions
+
+The main branch contains the latest stable version of the benchmarking library;
+the API of which can be considered largely stable, with source breaking changes
+being made only upon the release of a new major version.
+
+Newer, experimental, features are implemented and tested on the
+[`v2` branch](https://github.com/google/benchmark/tree/v2). Users who wish
+to use, test, and provide feedback on the new features are encouraged to try
+this branch. However, this branch provides no stability guarantees and reserves
+the right to change and break the API at any time.
+
 
 ## Example usage
 ### Basic usage