doc: Update MAINTAINERS.md with Symantec Whitelisting
doc: Update MAINTAINERS.md with Symantec Whitelisting

This adds instructions for submitting Etcher for false positive detection
to Symantec Endpoint Protection.

Change-Type: patch

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