docs: add codeclimate badge (#34)

Author: patrickmichalina

README.md

@@ -16,6 +16,9 @@
   <a href="https://david-dm.org/patrickmichalina/typescript-monads?type=dev">
     <img alt="greenkeeper" src="https://david-dm.org/patrickmichalina/typescript-monads/dev-status.svg">
   </a>
+  <a href="https://codeclimate.com/github/patrickmichalina/typescript-monads/maintainability">
+    <img alt="codeclimate" src="https://api.codeclimate.com/v1/badges/f40c9fff2927e49c3ea2/maintainability">
+  </a>
   <a href="https://github.com/semantic-release/semantic-release">
     <img alt="semantic-release" src="https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg">
   </a>