Add documentation about RUSTFMT environment variable

[topecongiro]

• Update README.md
• Update cargo's documentation. We should do this after we release the version with RUSTFMT support.

Related: #4419

[MonliH]

When is the new version going to be released? When it does, I could help.

[calebcartwright]

When is the new version going to be released?

Have to get a few other PRs landed in other repos since the next version contains a bump of the rustc-ap-* crates. Will update here once we have a more concrete answer

When it does, I could help.

Fantastic, thank you!

[calebcartwright]

rustfmt 1.4.22 has landed on nightly (and I believe beta now as well) so this should be good to go @MonliH if you are still interested in working on it

[MonliH]

Yep, sure! Will do.