Documentation: mention rustlings lsp in the initial ReadMe.md

[alelom]

See discussion in #443 (comment)

It would be useful to mention rustlings lsp in the main ReadMe.md of Rustlings. As a total beginner that decided to start his Rust journey with Rustlings, I only guessed that I was missing key functionality of the rust-analyzer Intellisense after a few days (and almost got thinking it was a poor analyzer)!

I only discovered rustlings lsp by digging in that issue, and only after I guessed I was missing key functionality after reading VsCode Rust documentation.

[mo8it]

It is already in the README.
But I totally get this problem. Rustlings has been redesigned in version 6 (still in beta) so that the language server works out of the box :)