add readme / warning for 4.x series

Signed-off-by: Nico Schottelius <nico@bento.schottelius.org>

README.4

@@ -0,0 +1,24 @@
+This branch contains experimental features for cdist 4.x:
+
+    - install support
+    - preos support
+
+They are not yet stable:
+
+    - use them at your own risk
+    - all __install types may change at any time (syntax, parameter, etc.)
+    - explorers for install may be broken
+
+    - core code is based on the master branch, but
+      contains changes for install and preos feature
+
+
+In short:
+
+                        _                                                         _     _    
+ _   _ ___  ___    __ _| |_   _   _  ___  _   _ _ __    _____      ___ __    _ __(_)___| | __
+| | | / __|/ _ \  / _` | __| | | | |/ _ \| | | | '__|  / _ \ \ /\ / / '_ \  | '__| / __| |/ /
+| |_| \__ \  __/ | (_| | |_  | |_| | (_) | |_| | |    | (_) \ V  V /| | | | | |  | \__ \   < 
+ \__,_|___/\___|  \__,_|\__|  \__, |\___/ \__,_|_|     \___/ \_/\_/ |_| |_| |_|  |_|___/_|\_\
+                              |___/                                                          
+