Document Minimum Go Version (#111)

Author: hiranya911

README.md

@@ -39,6 +39,11 @@ Please refer to the [CONTRIBUTING page](./CONTRIBUTING.md) for more information
 about how you can contribute to this project. We welcome bug reports, feature
 requests, code review feedback, and also pull requests. 
 
+## Supported Go Versions
+
+We support Go v1.7 and higher.
+[Continuous integration](https://travis-ci.org/firebase/firebase-admin-go) system
+tests the code on Go v1.7 through v1.10.
 
 ## Documentation