docs: contrib/issue templates

[henryiii]

This sets up some GitHub specifics to make the main pages a little cleaner, add nice issue templates and a prominent link to the gitter, and adds information requested by @rwgk about setting up development environments and building with CMake.

A second PR can be made to move CONTRIBUTING.md to .github/CONTRIBUTING.md , to save a little space on the main page. We can add a nice link to it from README.md.

[YannickJadoul]

Is it urgent to move CONTRIBUTING.md to .github? I've often seen it at the root?

Apart from that, I'm OK with this, but this isn't too urgent and I can imagine @wjakob wants to have a say/some input in this? So I'd suggest waiting before merging this until we can get some feedback?

[henryiii]

It was @wjakob's suggestion that we clean up the main folder a bit, though have no idea if likes CONTRIBUTING.md in the main folder or not. I am slightly in favor of the .github location for it, since it still shows up in all the proper GitHub prompts (that's true in either place) and keeps the main folder slightly cleaner. We can always link to it from the README, etc., if it needs to be more obvious.

No rush, agreed.

[YannickJadoul]

(Just assigning to @wjakob, so he can have a clear overview, in a few weeks)

[rwgk]

Thanks Henry, this looks great to me, just a few small suggestions.

How much trouble is it to move CONTRIBUTING.md in a separate PR: a pure move no changes?
That way it's easiest to see what changed.

[henryiii]

I've moved it back - we can move it later.

[wjakob]

Looks all good to me, and moving the github-related markdown files to a .github sub-directory is great.

[henryiii]

I'll do the move in a separate PR.