Create contributors guide

[kmannislands]

I have been looking in to contributing to ShareDB lately, but I didn't see a contributors guide or anything of the like.
Would have been/would still be helpful.
I am willing to author/template out one and make a PR with some guidance from the core team.

[kmannislands]

Any feedback here? @alecgibson @gkubisa

[alecgibson]

I'm sure it'd be useful if you want to put one together! I myself am very new to contributing to this project, so could do with learning a thing or two myself. Happy to give whatever you write a read.

[kmannislands]

@alecgibson Gotcha. I saw you a lot in the recent commit history so I figured you were the one to ask. Are there any full-time maintainers or other core stakeholders I should consult here?

[alecgibson]

@nateps and @ericyhwang are org owners I've spoken to, who may have opinions on a guide, but they can be hard to get a hold of because they're quite busy. There's a weekly pull request meeting you can join if you have things you want to discuss. Some more context around maintainers: #163

[curran]

Duplicate of #80

I'd vote for closing #80 in favor of this one, as this one is more fresh.

My sentiment from #80

Having contributor guidelines would definitely be useful. I have made a number of pull requests, and I'm not sure why they are not being reviewed or merged. My guess is that it's a lack of time on the part of the maintainers, but I also wonder if I'm doing something wrong, or if something more is expected from outside contributors like me (e.g. a detailed explanation of why the change is required). Having clarity on what's expected of contributors would be great.

By the way I'm thrilled to see there's been plenty of recent activity PR-wise thanks to @ericyhwang and others!