add deprecation notice

[bcoe]

I feel bad ignoring all the open pull requests on this repo, I've added a deprecation notice that reflects reality:

• we're not entertaining new pull requests or issues, since the registry has migrated towards being a set of micro-services.
• we still want to give the community the data that they need, but this will gradually come in the form of other APIs.

[isaacs]

Looks good to me. Merge at will.