gemspec: Add post install message

vagrant-lxc.gemspec

@@ -17,4 +17,11 @@ Gem::Specification.new do |gem|
   gem.executables   = gem.files.grep(%r{^bin/}).map{ |f| File.basename(f) }
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
   gem.require_paths = ["lib"]
+
+  gem.post_install_message = %Q{
+  Thanks for giving vagrant-lxc #{Vagrant::LXC::VERSION} a try!
+  This version introduces many changes and includes some deprecations,
+  please see the project's CHANGELOG:
+    https://github.com/fgrehm/vagrant-lxc/blob/master/CHANGELOG.md
+  }
 end