[DEV] open new 1.6 development branch

This new branch is based on 1.5.0, which 1.6-dev0 is 100% equivalent to.
The README has been updated to mention that it is a development branch.

Released version 1.6-dev0 with the following main changes :
    - exact copy of 1.5.0
diff --git a/VERSION b/VERSION
index bc80560..0b9d9ed 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.5.0
+1.6-dev0