Add a contexts/README.md to explain the DID context versioning scheme.

[dmitrizagidulin]

Add an explanation and changelog for the DID @context versioning scheme.

(Related to PR #270 and others.)

[peacekeeper]

Seem like a very useful README to me that finally explains the considerations around managing a JSON-LD context for DIDs.

Also looks consistent with @rhiaro 's recent PR #246 and comment #241 (comment).

[OR13]

As I noted on #270

Changes to the context, may cause broken links such as https://web-payments.org/vocabs/security#publicKeyJwk

It would be awesome to add a sentence or two about whether that matters or not, if it does matter, contributors should be guided to submit corrections for example:

"sec": "https://w3id.org/security#",

They can read more about how to accomplish that here: https://github.com/perma-id/w3id.org

[brentzundel]

This repo is scheduled to be archived. The work has moved to the DID WG.
The artifacts in the DID WG repository share commit history with this repo, so it should be possible to raise this PR against that repo.

[msporny]

Migrated to w3c/did-core#91. Closing.