Add deprecation notice

[WadeBarnes]

Should the repo be marked read-only as well?

That would be a good idea, once we merge this PR. I would like a couple more eyes on what I wrote first.

[WadeBarnes]

@TimoGlastra, @dhh1128, I've replaced the indy-shared-rs section with anoncreds-rs as suggested. Please have another quick look.

[TimoGlastra]

Changes look good to me!

[dhh1128]

LGTM