Move CONTRIBUTING.md to rustc-dev-guide

[mark-i-m]

cc rust-lang/compiler-team#296 @ecstatic-morse @rust-lang/compiler

This PR moves CONTRIBUTING.md to the rust-dev-guide. It should not merge before #731. The objective is to point all new contributors in one place. In the contributor survey, a surprisingly high number of people (more than half) pointed to CONTRIBUTING.md as a major resource they use to contribute to the compiler, whereas the new "Getting Started" guide is likely a better place to start.

That said, the content in CONTRIBUTING.md is still useful, so we move it here and take the opportunity to also move some other contributing-related chapters together in the guide.

Feedback is welcome.

[nikomatsakis]

Seems reasonable as a start. I know we've had some debates about where to put "contributing" vs "policies" vs "explaining how the code works" vs "how to build the compiler", and I still feel some uncertainty around that. I know that when I come to the rustc-dev-guide these days, the table of contents itself starts to feel a bit "intimidating" somehow.

I'm usually looking to give somebody a link to explain how some part of the compiler works, and so maybe part of it is that this material doesn't start until fairly far down. I think that having more headings with fewer subbullets would probably help here. (i.e., I approve of this organizational change)

[LeSeulArtichaut]

Can't find any reference to stable-* labels for stable backports. Did I miss it?

[mark-i-m]

I just copy/pasted the document with very minor edits. I don't know much about triage/tagging, so it might be out of date. Feel free to propose additions.