Update note about current version status.

README.md

@@ -1,6 +1,6 @@
 # Secure Headers [![Build Status](https://travis-ci.org/twitter/secureheaders.svg?branch=master)](http://travis-ci.org/twitter/secureheaders) [![Code Climate](https://codeclimate.com/github/twitter/secureheaders.svg)](https://codeclimate.com/github/twitter/secureheaders) [![Coverage Status](https://coveralls.io/repos/twitter/secureheaders/badge.svg)](https://coveralls.io/r/twitter/secureheaders)
 
-**master represents 5.x line**. See the [upgrading to 4.x doc](docs/upgrading-to-4-0.md) and [upgrading to 5.x doc](docs/upgrading-to-5-0.md) for instructions on how to upgrade. Bug fixes should go in the 3.x branch for now.
+**master represents 6.x line**. See the [upgrading to 4.x doc](docs/upgrading-to-4-0.md), [upgrading to 5.x doc](docs/upgrading-to-5-0.md), or [upgrading to 6.x doc](docs/upgrading-to-6-0.md) for instructions on how to upgrade. Bug fixes should go in the 5.x branch for now.
 
 **The [3.x](https://github.com/twitter/secureheaders/tree/2.x) branch is moving into maintenance mode**. See the [upgrading to 3.x doc](docs/upgrading-to-3-0.md) for instructions on how to upgrade including the differences and benefits of using the 3.x branch.