doc, enh: update description in citre.el

citre.el

@@ -1,4 +1,4 @@
-;;; citre.el --- Ctags IDE on the True Editor -*- lexical-binding: t -*-
+;;; citre.el --- Superior code reading & auto-completion tool with pluggable backends. -*- lexical-binding: t -*-
 
 ;; Copyright (C) 2020 Hao Wang
 
@@ -27,6 +27,10 @@
 
 ;;; Commentary:
 
+;; Citre is a superior code reading & auto-completion tool with pluggable
+;; backends.  Tags file backend, GNU global backend and eglot backend are
+;; bundled with Citre.
+
 ;; Read README.md to know more about Citre.  It also points you to a detailed
 ;; user manual.  If you haven't received such a file, please visit
 ;; https://github.com/universal-ctags/citre.