[Docs] Add reference to LSP to quickstart section

This PR adds reference to RuboCop's built-in LSP to quickstart section in the README.

While I recognize that the quickstart section is expanding,
I think that due to its potential to dramatically change the development experience,
LSP is worth mentioning as a second form of magic.

To keep the quickstart text concise, I have not mentioned the required RuboCop version.
It can be easily found by referring to the linked page.

README.md

@@ -67,6 +67,8 @@ $ cd my/cool/ruby/project
 $ rubocop
 ```
 
+You can also use this magic in your favorite editor with RuboCop's [built-in LSP](https://docs.rubocop.org/rubocop/usage/lsp.html).
+
 ## Documentation
 
 You can read a lot more about RuboCop in its [official docs](https://docs.rubocop.org).