chore: add versioning section to RELEASE.md and a link in README.md

[cbmarcum]

Refs: #14959

[jdaugherty]

Great Job @cbmarcum. While this explains our versioning scheme, it does not address ASF 'release' confusion.

@jamesfredley @matrei Should we add something that clarifies why we put out releases this way? I think @jamesfredley linked to Spring's guide as one example; should we mention that we base our versioning off Spring too? I'm also ok with only adding this and addressing these concerns outside of the code.

[jamesfredley]

I think you can get away with just the following here, which saves us from adjusting the branch going forward.

[RELEASE.md Appendix: Versioning](RELEASE.md#appendix-versioning)

[cbmarcum]

Thanks, I'll make the adjustments.

[jamesfredley]

I think adding a sentence stating that we follow the Spring release versioning scheme, with or without link, and that we release soon after each Spring Boot release, would add clarity. And a sentence distinguishing between a RCX release and a release candidate from the ASF release process perfective.

[cbmarcum]

@jamesfredley, @jdaugherty, and All, let me know what you think of the modifications.