update release procedure

[EdVinyard]

1. Why does yarn version add version "0.0.0" to the root package.json? Why do we have to manually remove it to get the release process to work? See Slack thread for details.
   • @daniel-white figured out that the docs should specify yarn run version, a lerna command, rather than yarn version which isn't the same thing.
2. Update the procedure described in CONTRIBUTING.md to...
   • match the current release procedure
   • include demo release conditions and instructions