docs(readme): add CLA Assistant badge

Author: mediumTaj

README.md

@@ -6,6 +6,7 @@
 [![codecov.io](https://codecov.io/github/watson-developer-cloud/ruby-sdk/coverage.svg)](https://codecov.io/github/watson-developer-cloud/ruby-sdk)
 [![Gem Version](https://badge.fury.io/rb/ibm_watson.svg)](https://badge.fury.io/rb/ibm_watson)
 [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg)](https://github.com/semantic-release/semantic-release)
+[![CLA assistant](https://cla-assistant.io/readme/badge/watson-developer-cloud/ruby-sdk)](https://cla-assistant.io/watson-developer-cloud/ruby-sdk)
 
 Ruby gem to quickly get started with the various [IBM Watson][wdc] services.