Describe configuration option 'autorun_database_cleaner' in README

Issues #215 and #232 resulted in adding a configuration option to
disable the by-default-usage of DatabaseCleaner:

Cucumber::Rails::Database.autorun_database_cleaner = false

This has to go into the README so that users can easily learn about
it. Currently Google finds the related issues first, which is not
optimal.
Merge pull request #255 from meismann/master

Describe configuration option 'autorun_database_cleaner' in README