Add warning about client URI dependence

This modification was inspired by https://wikia-inc.atlassian.net/browse/PLATFORM-607. Some of our mobile app clients rely upon the formatting of thumbnail URL in order to generate and display thumbnails that are appropriate for the device. This creates a coupling between the client and server which makes independent evolution of the server more difficult. That is, we can't change the thumbnail URLs without also changing the clients. We should try and avoid this situation in the future.

/cc @nmonterroso @aniaMu @pchojnacki