Document how to add terms locally

[MattiSG]

There is currently no way for a contributor to know how to concretely add terms: the only available links are to guidelines, and these do not describe how to write a JSON file, for example.

Replace that section in the README by a dedicated CONTRIBUTING file, which will explain to run npm install, npm test, link to the documentation for the specific usage of the CLI and guidelines, and suggest to open pull requests.