Document the CI infrastructure

[timsnyder]

CI is currently "documented" in a series of github issues and PRs where changes were made.

There's now been enough stuff done, that it would benefit from having a short one-page howto guide.

@knute-sifive , should I create the page as CI.md in the toplevel of the repo or a page in https://github.com/sparcians/map/wiki or somewhere else?

My first inclination would be to create a CI.md or README_CI.md in the toplevel of the repo so that people come across it easily.

[ghost]

That would be great. I like the README_CI.md concept. Thank you!

[klingaard]

Of course, making a wiki page would be super awesome too.