Improve CONTRIBUTING.md for first-time steps (#2901)
Improve CONTRIBUTING.md for first-time steps (#2901)

Upon first pull of the repository, the steps specified in
CONTRIBUTING.md to get started were not enough to actually
load the `docs` package and begin hacking away. This updates
those steps to bring the documentation up to snuff.
draft-js-plugins/draft-js-plugins,draft-js-plugins/draft-js-plugins,draft-js-plugins/draft-js-plugins,draft-js-plugins/draft-js-plugins