message: docs(RELEASE): Add documentation on how to make a release for maintainers of this package.
diff: - To release a new version of itk.js on npmjs:
  +Verify the source tree.
  +
  +```
  +git checkout master
  +git pull upstream master
  git clean -fdx
  npm install
  npm run build
  npm run test
  +```
  +
  +Bump `version` in `package.json`.
  +
  +version=6.0.0 # change to correct version
  +git commit -m "feat(version): Bump NPM version to ${version}"
  +cp LICENSE README.md package.json dist/
  +cd dist
  +npm publish
  +git tag -m "itk.js ${version}" -s v$version HEAD
  +git push --tags upstream master
  