chore(docs): added link to the badge release (ethereum#1401)

* Update README.md

Author: Olexandr88

README.md

@@ -1,6 +1,6 @@
 # Execution Spec Tests
 
-![latest version](https://img.shields.io/github/v/release/ethereum/execution-spec-tests)
+[![latest version](https://img.shields.io/github/v/release/ethereum/execution-spec-tests)](https://github.com/ethereum/execution-spec-tests/releases/latest)
 
 [ethereum/execution-spec-tests](https://github.com/ethereum/execution-spec-tests) is both a collection of test cases and a framework implemented in Python to generate tests for Ethereum execution clients.