Add a deprecation warning on usage of `Slavery.spec_key=` interface

[asanghi]

This needs to go on a separate branch v2.2.0 not on master branch?

[kenn]

Hmm, as it is not a "feature", I'm not keen to release as minor version increment. It doesn't add much value.

On second thought, probably it's ok to just go with the README - it's a major upgrade anyway, and you would expect something might not work.

What do you think?