Update README.rdoc

If default mysql adapter is not present, it gives this error
Please install the mysql adapter: `gem install activerecord-mysql-adapter` (Could not find mysql (~> 2.8.1) amongst[...]
While it needs mysql gem to be installed, and not activerecord-mysql-adapter. Since mysql2 is better than mysql gem, this should be used by default.
Merge pull request #14 from ketan21/patch-1

Update README.rdoc