Add note about v1 and v2 branches to README

Author: mbrubeck

README.md

@@ -1,6 +1,18 @@
 rust-smallvec
 =============
 
+> **Note**
+> This is the code for smallvec 2.0, which is not yet ready for release.  For
+> details about the changes in version 2.0, please see #183, #240, and #284.
+>
+> The source code for the latest smallvec 1.x.y release can be found on the
+> [v1 branch].  Bug fixes for smallvec 1 should be based on that branch, while
+> new feature development should go on the v2 branch.
+
+[v1 branch]: https://github.com/servo/rust-smallvec/tree/v1
+
+## About smallvec
+
 [Documentation](https://docs.rs/smallvec/)
 
 [Release notes](https://github.com/servo/rust-smallvec/releases)