README: note how to enable debugging for rustc

[PlasmaPower]

I got stuck on this for a bit, looking for a debug option in ./x.py build --help.

Diff without newline changes:

Before:

Various other options are also supported, and are documented in the config file.

After:

Various other options, such as enabling debug information, are also supported, and are documented in the config file.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[PlasmaPower]

I just noticed it's in CONTRIBUTING.md: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#rust

It still might be nice to have in the readme though - I didn't think to look in contributing at first.

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit 857d9db has been approved by Mark-Simulacrum