Updated README.md with link to Code Reviews

Updated `README.md` with link to Code Reviews section under `CONTRIBUTING.md`

Author: Pawka

README.md

@@ -156,6 +156,7 @@ Quick links:
 - [Bug Reports](https://github.com/sendgrid/php-http-client/blob/master/CONTRIBUTING.md#submit-a-bug-report)
 - [Sign the CLA to Create a Pull Request](https://github.com/sendgrid/php-http-client/blob/master/CONTRIBUTING.md#cla)
 - [Improvements to the Codebase](https://github.com/sendgrid/php-http-client/blob/master/CONTRIBUTING.md#improvements-to-the-codebase)
+- [Review Pull Requests](https://github.com/sendgrid/php-http-client/blob/master/CONTRIBUTING.md#code-reviews)
 
 <a name="thanks"></a>
 # Thanks