Warn contributors about the effect of patches on revisions

When preparing patches, I knew that head.hackage would prefer the versions it has patches for, even if the patches are empty. However, I was unaware of the consequences it has for hackage revisions.

A patch for a package prevents hackage revisions from being used in head.hackage for that package, meaning that one might end up contributing a patch that only updates dependency bounds on the unrevised package, when revisions with those same bounds are already available in hackage.

Perhaps the section on preparing patches could warn of this nuance, and advice to check if patches that only modify dependency bounds have revisions with the updates already in hackage.