docs(CLI.md): write the CLI.md document (#511)
docs(CLI.md): write the CLI.md document (#511)

This document explains what the Etcher CLI is, how can be used, and some
peculiarities, like `--robot` and exit codes.

Signed-off-by: Juan Cruz Viotti <8493db2a4f45d66df375afe12f7eba59fe1c8d78@gmail.com>
resin-io/etcher,resin-io/resin-etcher,resin-io/herostratus,resin-io/etcher,resin-io/etcher,cedricve/etcher,resin-io/etcher,resin-io/resin-etcher,cedricve/etcher,resin-io/etcher,resin-io/herostratus,cedricve/etcher