Consider adding a contribute.json

This was pointed out by https://observatory.mozilla.org/analyze/profiler.firefox.com :

From https://infosec.mozilla.org/guidelines/web_security#contributejson :

contribute.json is a text file placed within the root directory of a website that describes what it is, where its source exists, what technologies it uses, and how to reach support and contribute. contribute.json is a Mozilla standard used to describe all active Mozilla websites and projects.

Its existence can greatly speed up the process of bug triage, particularly for smaller websites with just a handful of maintainers. It further assists security researchers to find testable websites and instructs them on where to file their bugs against. As such, contribute.json is mandatory for all Mozilla websites, and must be maintained as contributors join and depart projects.

[canova]

The schema doesn’t have a field for slack but even the examples in the website include some fields that’s not in the schema. So, I believe we can add a field like "slack": "https://devtools-html-slack.herokuapp.com/" inside participate object.

[Manish-Giri]

@canaltinova Thank You! I just submitted the PR for this issue.

Please let me know if you’re aware of any other issues that I could work on, which would be fairly simple for beginning contributors!