Update CONTRIBUTING.md

CONTRIBUTING.md

 Be nice:
 
- * fork the repo
- * work in a branch of your fork
- * make a pull request to merge your branch to the master
+ * Please **do not** make changes to the repo master so:
+    * fork the repo
+    * work in a branch of your fork
+    * make a pull request to merge your branch to the master
+ 
+Don't know how to do this? [Read our guide](gitBranches.md).
  
- Please **do not** make changes to the repo master
\ No newline at end of file