Added note about setting the cassette_library_dir in Rails.

I ran into this issue the first time I was setting up vcr, and thought that others may benefit from having it in the docs.

https://relishapp.com/vcr/vcr/v/2-4-0/docs/configuration/cassette-library-dir!

Note: When using Rails, avoid using the `test/fixtures` directory 
 to store the cassettes. Rails treats any YAML file in the fixtures 
 directory as an ActiveRecord fixture.
 This will cause an `ActiveRecord::Fixture::FormatError` to be raised.