Added note about [ci skip] tag to contributing guide

railties/guides/source/contributing_to_ruby_on_rails.textile

@@ -271,6 +271,8 @@ When working with documentation, please take into account the "API Documentation
 
 NOTE: As explained earlier, ordinary code patches should have proper documentation coverage. docrails is only used for isolated documentation improvements.
 
+NOTE: To help our CI servers you can add [ci skip] tag to your documentation commit message to skip build on that commit. Please remember to use it for commits containing only documentation changes.
+
 WARNING: docrails has a very strict policy: no code can be touched whatsoever, no matter how trivial or small the change. Only RDoc and guides can be edited via docrails. Also, CHANGELOGs should never be edited in docrails.
 
 h3. Contributing to the Rails Code