Added instruction to use 'gem "cgi"' or Bundler

Author: hsbt

lib/cgi.rb

@@ -3,5 +3,7 @@
 require "cgi/escape"
 warn <<-WARNING, uplevel: Gem::BUNDLED_GEMS.uplevel if $VERBOSE
 CGI library is removed from Ruby 4.0. Please use cgi/escape instead for CGI.escape and CGI.unescape features.
-If you need to use the full features of CGI library, Please install cgi gem.
+
+If you need to use the full features of CGI library, please add 'gem "cgi"' to your script
+or use Bundler to ensure you are using the cgi gem instead of this file.
 WARNING