Provide guidance on working with out-of-tree crates

[davidtwco]

It would be worthwhile to document all of the best practices surrounding working with out-of-tree crates and the compiler.

This issue exists to be linked to by the crates policy in the compiler-team repository.
Related to #268.

[mark-i-m]

@davidtwco Would you be interested in starting such a chapter?

[davidtwco]

@mark-i-m I don't have any experience working with out-of-tree crates, so unfortunately I don't know what the best practices to document are. Someone who has worked on chalk or polonius would be in a good position to document these.

[mark-i-m]

@tmandry @scalexm or @nikomatsakis (worked on chalk), and I know that @RalfJung and @oli-obk work on miri, and @Mark-Simulacrum works on infra. Perhaps one of them has some info or would be willing to type up a short PR (it would be much appreciated <3)?

[jyn514]

@davidtwco do you think the new chapter I added in #1653 (which is really just moving around info in contributing.md today) documents this well enough we could close the issue?

[davidtwco]

@davidtwco do you think the new chapter I added in #1653 (which is really just moving around info in contributing.md today) documents this well enough we could close the issue?

Yeah, I think this looks good.