Document RUSTFMT environment variable (#4464)

* Document RUSTFMT env var

* Move documentation up

* Apply suggestions from code review

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

* Fix accedental removal

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

Author: MonliH

README.md

@@ -102,6 +102,25 @@ read data from stdin. Alternatively, you can use `cargo fmt` to format all
 binary and library targets of your crate.
 
 You can run `rustfmt --help` for information about available arguments.
+The easiest way to run rustfmt against a project is with `cargo fmt`. `cargo fmt` works on both
+single-crate projects and [cargo workspaces](https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html).
+Please see `cargo fmt --help` for usage information.
+
+You can specify the path to your own `rustfmt` binary for cargo to use by setting the`RUSTFMT` 
+environment variable. This was added in v1.4.22, so you must have this version or newer to leverage this feature (`cargo fmt --version`)
+
+### Running `rustfmt` directly
+
+To format individual files or arbitrary codes from stdin, the `rustfmt` binary should be used. Some
+examples follow:
+
+- `rustfmt lib.rs main.rs` will format "lib.rs" and "main.rs" in place
+- `rustfmt` will read a code from stdin and write formatting to stdout
+  - `echo "fn     main() {}" | rustfmt` would emit "fn main() {}".
+
+For more information, including arguments and emit options, see `rustfmt --help`.
+
+### Verifying code is formatted
 
 When running with `--check`, Rustfmt will exit with `0` if Rustfmt would not
 make any formatting changes to the input, and `1` if Rustfmt would make changes.