Add contributing doc for eyre

[yaahc]

Heavily inspired by https://github.com/bevyengine/bevy/blob/main/CONTRIBUTING.md

Fixes #98

• Rendered

I wanted to keep the docs minimal to start and grow them organically based on need rather than including everything I can think of and making it overwhelming for people, but I'm unsure where to strike the balance. There's a bunch of good stuff from Bevy's contribution guide that I cut that I would love to add over time based on feedback from other contributors.

I think the biggest question right now is how to handle issue tags. Bevy has a comprehensive system similar to the Rust project's issue tags. I want to ensure we have the tags we need to keep things organized, but I'm not sure that we need anything that robust; we don't have nearly as much traffic. I'm leaning towards starting with a minimal set of tags inspired by bevy. The stuff about status (e.g. S-controversial) and difficulty and how those relate to reviews seem like they could be pretty useful for onboarding new reviewers and contributors.

I'll make sure to add all the right tags and check that all the links are working before merging this.

Long term I want to update this or create companion docs to describe various roles like "Triage", "Review", "Merge", etc which may entail rewriting decent chunks of this.

[pksunkara]

Looks good on first look.

Approving assuming that label and PR examples are moved away from bevy.

[ten3roberts]

Looks fantastic.

Really like the clear focus on the fact that anyone is free to contribute and you don't need to be an unsafe Rust zealot to do so.

The part on documentation and constructive criticism is also splendid.

I assume the specific labels will be changed to ones specific to eyre in time.

[ten3roberts]

Additionally, just confirming that I am absolutely fine to @mention in PRs. Sometimes you may miss a message due to the volume of mails, so the explicit mention always catches my eye.