Add contributing and license sections to README

Author: komagata

README.md

@@ -100,3 +100,11 @@ new TextareaMarkdown(textarea, {
   ]
 })
 ```
+
+## Contributing
+
+Bug reports and pull requests are welcome on GitHub at https://github.com/komagata/textarea-markdown
+
+## License
+
+The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).