Add README for publishing mobile apps on `meteor build`

1. Add README files for both the ios/ and android/
   subdirectories of a build directory linking to a
   Wiki page with instructions on how to go through
   the publish process for both app stores.

2. To make the README file more visible in iOS projects,
   and to make the ios directory tree mirror the android
   one, place what used to go in <BUILD DIR>/ios/ into
   <BUILD DIR>/ios/project/. <BUILD DIR>/ios/ now only
   contains the README file.