docs(lsp): mention +booster

Ref: #8463

modules/tools/lsp/README.org

@@ -21,6 +21,10 @@ for its =+lsp= flag).
 /This module has no dedicated maintainers./ [[doom-contrib-maintainer:][Become a maintainer?]]
 
 ** Module flags
+- +booster ::
+  Speed up I/O between LSP clients and Emacs by converting JSON output from LSP
+  servers into Elisp bytecode. Only works for Eglot, at the moment, and requires
+  [[https://github.com/blahgeek/emacs-lsp-booster][emacs-lsp-booster]] (also needed on remote systems if editing over TRAMP).
 - +eglot ::
   Use [[https://elpa.gnu.org/packages/eglot.html][Eglot]] instead of [[https://github.com/emacs-lsp/lsp-mode][LSP-mode]] to implement the LSP client in Emacs.
 - +peek ::