Update Documentation/README.md to cover more files. (#928)

Update Documentation/README.md to cover the Proposals and ABI
directories.

I have also copied my forum post regarding #162 and #840 into a Markdown
document, but haven't added it to README.md as it's prospective only and
hasn't been accepted as either a vision document nor formal proposal.

I have _also_ deleted Releases.md as obsolete now that our releases are
tied to the Swift project's releases.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan