Added source attribution

Author: Kotsias, Panagiotis-Christos

README.md

@@ -1,4 +1,7 @@
 [![Build Status](https://travis-ci.com/pcko1/etherscan-python.svg?branch=master)](https://travis-ci.com/pcko1/etherscan-python) 
 [![codecov](https://codecov.io/gh/pcko1/etherscan-python/branch/master/graph/badge.svg)](https://codecov.io/gh/pcko1/etherscan-python)
 
-Minimal python API for etherscan.io.
+Minimal python API for [etherscan.io](etherscan.io).
+
+___
+Powered by Etherscan.io APIs.