readme: Update for latest continuous integration changes

[nathanchance]

ClangBuiltLinux/continuous-integration2#679
ClangBuiltLinux/continuous-integration2#682

[JustinStitt]

Yes!

Although, I wonder if there's a better way to keep this README synced up with our matrix from https://github.com/ClangBuiltLinux/continuous-integration2.

Either way LGTM!

[nathanchance]

Although, I wonder if there's a better way to keep this README synced up with our matrix from https://github.com/ClangBuiltLinux/continuous-integration2.

We could potentially look into if this is possible with GitHub Actions... but the fact it is two separate repos may complicate things. The trees do not change all too often, it is really just the LTS cycle getting us here. Maybe we should have some sort of check list or how to that we go through for tree additions or removals, that would potentially be helpful for more than just us.