Add contributing section?

[spastorino]

We should have a contributing section on https://rust-lang.github.io/compiler-team/.

It could say:

• sign up for zulip
• check out the rustc-guide
• look for easy, mentor issues
• join an ICE-breaker group
• follow along with a working group
• could also mention #t-compiler/help-wanted once it exists

Do we have content like this on rustc-guide?, maybe we should just have a contributing section that redirects there?.

rustc-guide should be the canonical home for that sort of thing, as it can go into a lot more depth, and we have a lot of that content.