Add post_install_message

Author: aycabta

readline.gemspec

@@ -17,6 +17,16 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['BSDL', 'COPYING', 'README.md', 'lib/readline.rb']
   spec.require_paths = ['lib']
 
+  spec.post_install_message = <<~EOM
+    This is just loader for "readline". If Ruby has "readline-ext" gem that
+    is a native extension, this gem will load its first. If Ruby doesn't have
+    the "readline-ext" gem this gem will load "reline" that is a compatible
+    library with "readline-ext" gem and is implemented by pure Ruby.
+
+    If you intend to use GNU Readline by `require 'readline'`, please install
+    "readline-ext" gem.
+  EOM
+
   spec.add_runtime_dependency 'reline'
   spec.add_development_dependency 'bundler'
   spec.add_development_dependency 'rake'