[snowpatch] [PATCH v2 4/6] README.md: Update installation instructions

[Russell Currey]

On Mon, 2018-07-23 at 17:04 +1000, Andrew Donnellan wrote:
> Update the installation instructions in preparation for pushing a
> release
> to crates.io, which I intend to do very soon.
> 
> Also clarify that snowpatch will only run on platforms that support
> both
> Rust and Git, because I'm not sure snowpatch will run bare-metal on a
> microcontroller.
> 

Dare you to try...