Adding a contributor's guide

[febuiles]

Adding a CONTRIBUTING.md outlining the basic steps to run tests and do local development. It also includes instructions on how to release a new version, which could be helpful for other folks getting involved with the project.

This document was copy/pasted from another project and needs to be tweaked/improved for this specific tool, but I feel it's better than nothing.

TODO

• Cut a new version (1.2.7). The PR also includes a bump to the action's version and should be followed by a new release, 1.2.6 did not update the version field in package.json.