Update README.md

ask users to submit issues in readme file

Author: abahman

README.md

@@ -4,3 +4,5 @@ A python-based air conditioning and heat pump system model v1.5
 This version uses CoolProp v6.x
 
 The documention is available on ReadTheDocs: [![Go to the docs](https://img.shields.io/badge/docs-latest-brightgreen.svg?style=flat)](http://achp.readthedocs.io/en/latest/)
+
+Please help us improve ACHP model by submiting any issues [here](https://github.com/TSTK/ACHP/issues)