DOC: Update README to mention pending migration to NumPy

[person142]

Update the README to reflect the upcoming migration of the stubs
into the main NumPy repo. Hopefully this will help clarify the
plan for people interested in contributing.

[rgommers]

@person142 so you want to do this migration over the next days/weeks? And the open PRs, do they need to be dealt with first, or resubmitted after the migration?

[person142]

so you want to do this migration over the next days/weeks?

Yes, hopefully will start this weekend.

And the open PRs, do they need to be dealt with first, or resubmitted after the migration?

#80 is the remaining active one (other than a couple from me), so I would like to wrap that one up.

Other than that things are either stale (from 2018) or experimental PRs I made that need reworking anyway.

[rgommers]

Sounds good, thanks. In it goes.