Improve contributing text about subtrees

[jsoref]

https://rustc-dev-guide.rust-lang.org/contributing.html is really unhelpful.

There's a link for "External Dependencies", but:

1. I wouldn't guess that it'd mean "things we have embedded in our repository but should be treated as external dependencies"
2. it's just a link to https://rustc-dev-guide.rust-lang.org/external-repos.html

Beyond that, the final page doesn't actually answer my question of "where/how should changes be contributed".

The answer apparently being "In general such changes should be sent to the external repositories even if you find the code in /rust, unless you're making some other change to /rust and doing so would break those files, in which case your commit should include the fixes to them".