Expand README to include more information about features

[holocircuit]

I reorganised the README a tiny bit - this is not a large change.

Here are the important changes:

• I've added more information about certain functions, e.g. rust-run, rust-test, and gave an example of how to bind these to keys.
• I've removed the reference to straight.el, as that's more a general Emacs point than something to do with rust-mode
• I've explained that this package doesn't deal with auto-completion or jumping to definitions, and added a section pointing to using those. (The previous README referenced these but didn't explain why.)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brotzeit (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[mookid]

@holocircuit: let's merge that tomorrow evening, if you have nothing more to add?