lang/racket: add +lsp flag

And add link to racket's README.

modules/lang/racket/README.org

@@ -23,6 +23,7 @@ This module provides integration for [[https://github.com/greghendershott/racket
 This module has no dedicated maintainers.
 
 ** Module Flags
++ =+lsp= Enables LSP support. Requires jeapostrophe/racket-langserver.
 + =+xp= Enables the explore mode, which "analyzes expanded code to explain and explore."
 
 ** Plugins

modules/lang/racket/config.el

@@ -26,6 +26,9 @@
              #'rainbow-delimiters-mode
              #'highlight-quoted-mode)
 
+  (when (featurep! +lsp)
+    (add-hook 'racket-mode-local-vars-hook #'lsp!))
+
   (when (featurep! +xp)
     (add-hook 'racket-mode-local-vars-hook #'racket-xp-mode)
     ;; Both flycheck and racket-xp produce error popups, but racket-xp's are