Clarify that OS X docs are not for development [ci skip]

docs/developers-guide-osx.md

 # Metabase OS X App
 
+NOTE: These instructions are only for packaging a built Metabase uberjar into `Metabase.app`. They are not useful if your goal is to work on Metabase itself; for development, please see our [developers' guide](developers-guide.md). 
+
 ## Prereqs
 
 1.  Install XCode.