Improving the documentation around Factory Linting and the DatabaseCleaner class.

If you follow the "Linting Factories" instructions in your [GETTING_STARTED.md](https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.md#linting-factories) file, a Rails application will break with this error message:

`uninitialized constant DatabaseCleaner (NameError)`

This is because `DatabaseCleaner` is class defined in the `database_cleaner` gem.

After reporting this in [Issue #644](https://github.com/thoughtbot/factory_girl/issues/644), it was suggested that I could help by improving the documentation around Factory Linting and the DatabaseCleaner class in the `GETTING_STARTED.md` file. Hence this pull request :-]