Update CONTRIBUTING.md

CONTRIBUTING.md

 Be nice:
 
+ * 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
  
- Please **do not** make changes to the repo master
\ No newline at end of file
+Don't know how to do this? [Read our guide](gitBranches.md).
+