Add `red_knot/README.md` (#16230)
## Summary
Resolves #15979.
The file explains what Red Knot is (a type checker), what state it is in
(not yet ready for user testing), what its goals ("extremely fast") and
non-goals (not a drop-in replacement for other type checkers) are as
well as what the crates contain.
## Test Plan
None.
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
