Add notice about dangers of using master branch.

README.md

@@ -10,6 +10,11 @@
 
 Flexible and feature-complete [Redis](http://redis.io) client for PHP >= 5.3 and HHVM >= 2.3.0.
 
+__ATTENTION:__ you are on the README file of an unstable branch of Predis specifically meant for the
+development of future releases. This means that the code on this branch is potentially unstable, and
+breaking change may happen without any prior notice. Do not use it in production environments or use
+it at your own risk!
+
 Predis does not require any additional C extension by default, but it can be optionally paired with
 [phpiredis](https://github.com/nrk/phpiredis) to lower the overhead of the serialization and parsing
 of the [Redis RESP Protocol](http://redis.io/topics/protocol). For an __experimental__ asynchronous