Add some contributor docs for debugging testsuite tests.
Add some contributor docs for debugging testsuite tests.

nipunn1313/cargo,rust-lang/cargo,eminence/cargo,nipunn1313/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,rust-lang/cargo,rustyhorde/cargo