Add initial README.md for tests/ requirement files

[x1101]

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First pass at adding documentation requested in #312 I did this as a table because that's the format that made sense to me, but if other have a different approach, or what other information included, I'm happy to improve from here.

Fixes #312

[x1101]

@webknjaz can you take a look and see if that's in line with what you're suggesting both for the updates to the description on constraints.in and the formatting for the *.txt files?

[samccann]

The CI failures here I think are unrelated to this PR so.. .bear with while we chase down the problems.

[samccann]

gonna close/reopen to see if the CI problems go away

[webknjaz]

Could you replace "requirements" with "dependencies" in the table?

[x1101]

@oraNod I think I have all your suggestions applied correctly. I'd appreciate another once-over to confirm

[oraNod]

thinking about branching devel to cut new stable releases. certain of the workflow files are deleted and kept on devel only. the corresponding requirements files are also deleted, which means this doc would not reflect what is on the stable branches.

we could note this in the MAINTAINERS.md file and say to either modify or delete this table.

[samccann]

@x1101 @oraNod are we expecting more changes here? If not can you merge @oraNod as it has the 2 approvals.

[oraNod]

Thanks @x1101