docs: Add info about required npm version
docs: Add info about required npm version

Npm 3.10 version is required to install dev tools correctly on Linux.

Change-type: patch
Signed-off-by: amdomanska <47b3af9da9b3132ba7a10479a787862f50e1928b@resin.io>

resin-io/resin-etcher,resin-io/etcher,resin-io/etcher,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/resin-etcher,resin-io/etcher,resin-io/herostratus