Document the need of npm install --force
Document the need of npm install --force

This is needed to avoid optional dependencies not being installed.

Fixes: https://github.com/resin-io/herostratus/issues/22

cedricve/etcher,cedricve/etcher,resin-io/etcher,resin-io/etcher,resin-io/resin-etcher,resin-io/herostratus,resin-io/etcher,fedealconada/resin-etcher,resin-io/etcher,cedricve/etcher,fedealconada/resin-etcher,resin-io/etcher,resin-io/herostratus,resin-io/resin-etcher