Document RUSTFMT environment variable (#4464)
Document RUSTFMT environment variable (#4464)

* Document RUSTFMT env var

* Move documentation up

* Apply suggestions from code review

Co-authored-by: Caleb Cartwright <57164230d45de9cae60a8d520caf70bd9319ca42@users.noreply.github.com>

* Fix accedental removal

Co-authored-by: Caleb Cartwright <57164230d45de9cae60a8d520caf70bd9319ca42@users.noreply.github.com>
# Conflicts:
#	README.md

graydon/rust,graydon/rust,graydon/rust,rust-lang/rustfmt,SingingTree/rustfmt,aidancully/rust,SingingTree/rustfmt,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,rust-lang/rustfmt