Document how to install command line tool

[ssbarnea]

While the readme mentions the posibility to run from command line, there is zero information in the readme regarding how to install the cli on any platform.

[nicksnyder]

Can do:

go install github.com/sourcegraph/codenotify@latest

Then run the codenotify binary from the location go installed it (see go help install for info on where things are installed).

We could also set up https://goreleaser.com/intro/.