complete revision of the Contributing to Ruby on Rails guide

* adds instructions for PostgreSQL
* adds references to mysql2 here and there
* puts documentation contributions on par with code contributions
* adds a section about Rails Contributors
* documents C dependencies for Nokogiri and the database libs
* everything double-checked and revised step by step
* adds a section about warnings in the test suite
* removes section about the Rails wiki
* title and filename renamed
* overall rewording