docs: add Ale config to README

[coolaj86]

• Syntastic is deprecated and recommends upgrading to Ale
• Ale has supported rust.vim since Added linting of tests and examples for rust cargo linter dense-analysis/ale#1627

[rust-highfive]

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

Please see the contribution instructions for more information.

[coolaj86]

Thanks.

Side note: I just added rust.vim to webinstall.dev: https://webinstall.dev/vim-rust

(vim-rust in keeping with vim-* for the other installers for consistency and easy memorization, but it's also aliased properly as rust.vim too https://webinstall.dev/rust.vim)

[chapinb]

Hey all! Any reason not to merge this PR? I was able to use these instructions for configuration with ale.