* rustfmt install instruction gotcha
rustfmt stopped working for me. It turns out people started having trouble with this last year already https://github.com/rust-lang-nursery/rustfmt/issues/2304
* rustfmt on a single file now works! Secrets inside
After `rustup component add rustfmt-preview` I get rustfmt as a command in my terminal.
`rustfmt --help` reveals --write-mode [replace|overwrite|display|plain|diff|coverage|checkstyle] and we've been using the default which is either `overwrite` or `replace` (depending on rustfmt version) and this was changing a bunch of files and a messed up workflow.
* Newcomer-friendly explanation how to use `rustfmt`
