message: doc: need to `rustup upgrade` before override
diff: @@ -5,15 +5,25 @@ Rust userland library for Tock (WIP)
  This project is nascent and still under heavy development, but first steps:
  -1. Get a copy of the latest nightly, in this repo's root:
  +1. First, update rustup to ensure you have the latest toolchain available:
  +
  + `rustup upgrade`
  +
  +2. Get a copy of the latest nightly, in this repo's root:
  `rustup override set nightly`
  -2. Need to grab a copy of the rust sources:
  + Your rustc should be at least this new:
  + ```
  + $ rustc --version
  + rustc 1.21.0-nightly (7ac979d8c 2017-08-16)
  + ```
  +
  +3. Need to grab a copy of the rust sources:
  `rustup component add rust-src`
  -3. Now you should be able to build with:
  +4. Now you should be able to build with:
  `xargo build --target thumbv7em-tock-eabi`
  