DOC: create a BRANCHES file to explain the life cycle

This file tries to describe development versus maintenance and gives hints
about what version to pick depending on the user's skills and goals.
diff --git a/README b/README
index 915e038..a2c8b19 100644
--- a/README
+++ b/README
@@ -4,6 +4,7 @@
 Please refer to the following files depending on what you're looking for :
 
   - INSTALL for instructions on how to build and install HAProxy
+  - BRANCHES to understand the project's life cycle and what version to use
   - LICENSE for the project's license
   - CONTRIBUTING for the process to follow to submit contributions