Document conflicts in `db/schema.rb`

[Various](https://groups.google.com/forum/#!topic/git-users/_8nfgHonJm0) [opinions](http://www.saturnflyer.com/blog/jim/2010/09/14/always-check-in-schema-rb/) [on](http://tbaggery.com/2010/10/24/reduce-your-rails-schema-conflicts.html) how to resolve conflicts in `db/schem.rb` were given. But, while the Guides recommend the file is checked in source control, there's no official take on how to proceed. This PR suggests a solution and raises awareness to the issue.