Install a Blueprint validator as a dev dependency

[erunion]

https://github.com/Aconex/api-blueprint-validator-module is an NPM package that we use at Vimeo for validating our Blueprint files after Mill generates them, so it only makes sense to pull that into Mill to have the Blueprints that Mill can generate validate before we tag releases.

After this is in, also update the CONTRIBUTING file with details for anyone who wants to help out on the project.