feat: add asynchronous LSP plugin diagnostics and definitions

src/main.rs

@@ -34,7 +34,10 @@ Buffers: :edit PATH, :buffers, :buffer N, :bnext, :bprev, :bdelete[!] [N]
          :q refuses any dirty buffer; :wq saves active and quits only if all are clean
 
 Ruby config: $XDG_CONFIG_HOME/vamp/config.rb or ~/.config/vamp/config.rb
-Set VAMP_RUBY to choose the Ruby executable.";
+Set VAMP_RUBY to choose the Ruby executable.
+
+Optional LSP Ruby plugin: config.plugin PATH, options: { servers: [...] }
+LSP commands when enabled: :lsp_definition, :lsp_diagnostics";
 
 struct Options {
     file: Option<PathBuf>,