Add contributing guide

[chrysn]

This project should have had one way earlier.

• @kbarning and @Teufelchen1, would this document have made work on the UART wrappers easier?
• @jkarinkl, you've had your hands on a few texts on what contributing guides should look like – could this work?

[Teufelchen1]

It's on point! Do you think we should add a link to this document in the README.md?

[jkarinkl]

Nice @chrysn! Looks good.

In general a Contributing Guide has the goal to "increase the number of successful contributors to your project" (CNCF HowTo: Make a Contributing Guide).

Therefore it is handy to ask: what is the intended audience for this document? What is the background/level of knowledge/experience of these contributors, and thus what content should be added in the CG? (from @Teufelchen1's approval, I expect the content fits the audience here).

Some general things to check if they are relevant to add:

• Where and how can people ask for help and/or interact?
  • If this is different from the general channels people can use when contributing to RIOT
  • Also: is this question relevant, or do the people who want to contribute to this project already know regular ways to contact you/the maintainers of this project? Then it does not need to be mentioned in the CG.
• What kind of contributions are welcome?
  • Code only?
  • Documentation? (if relevant)
  • ...?
• When should PRs be submitted?
  • As work-in-progress
  • Or when finished?

[jkarinkl]

It's on point! Do you think we should add a link to this document in the README.md?

I would do so :).

[jkarinkl]

What sorts?

[chrysn]

I'd rather not limit peoples' imagination. People will have an idea what they want to do by the time they get to this document, and if someone wants to contribute an introduction poem, I'm happy to review metre.

[chrysn]

Thanks for your input, I've pushed commits to address those which I marked as resolved without a further comment. This includes a link to the matrix channel. (GitHub is an implied channel – but people read this so they already found that).

The audience this is intended for is people who already do consider contributing to the repository. For code contributions, which so far are the overwhelming majority (actually your response is probably the first non-code one 🥇), there's some implied level of experience; I wouldn't know how to put that in text without discouraging small fixes by newcomers. I'd trust the "common pitfalls" section (as well as recently introduced "Callback lifetimes are hard" in the tl;dr) to discourage completely inexperienced people from attempting to do large things (and when that does not work, chances are that a level of hubris is involved for which "some experience is expected" won't do any good either).