doc(CONTRIBUTING): Add webpack step to instructions
doc(CONTRIBUTING): Add webpack step to instructions

Change-Type: patch

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