Merge pull request #490 from netlify/docs/improve-release-docs

Improve `CONTRIBUTING.md`

Author: ehmicky

CONTRIBUTING.md

@@ -5,7 +5,7 @@ please read the [code of conduct](CODE_OF_CONDUCT.md).
 
 ## Releasing
 
-1. `npm version [major, minor, patch]`: generate changelog, bump version, version commit + tag.
+1. `npm version major`, `npm version minor`, `npm version patch` or `npm version prerelease`: generate changelog, bump version, version commit + tag.
 2. `npm run release`: push version commit + tag, push latest CHANGELOG entry to GitHub release page.
 3. Wait for Jenkins build to finish
 4. Create a PR in the [buildbot](https://github.com/netlify/buildbot) to bump [the version](https://github.com/netlify/buildbot/blob/a247edab7ead955cc27bb70ecc9f081e68f1aea6/script/docker-build.sh#L17) of the `build-image`.