Updated README.md to communicate MCW as MDL v2

[rystraum]

Since #5022 already being a duplicate of 2 other issues, I think it would be important for people to know this fact. There's nowhere else that documents this relationship.

I put in @Garbee's message to me almost verbatim, wording it as conservative as possible.

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[rystraum]

I signed it!

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CLAs look good, thanks!

[sgomes]

LGTM, thanks @rystraum!