Added OS X specific commands to installation guide [ci skip]
Recommended using homebrew for installing MySQL and PostgreSQL
'DefaultsTestWithoutTransactionalFixtures#test_mysql_text_not_null_defaults_non_strict' to pass when using MySql we needed to add select permissions to *.* [ci skip]

Otherwise we will get
ActiveRecord::StatementInvalid: Mysql::Error: Field 'non_null_text' doesn't have a default value: INSERT INTO `test_mysql_text_not_null_defaults` VALUES ()