Add note about required Go version to README

Closes #230.

README.md

@@ -2,7 +2,7 @@
 
 Linux: [![Build Status](https://travis-ci.org/golang/dep.svg?branch=master)](https://travis-ci.org/golang/dep) | Windows: [![Build status](https://ci.appveyor.com/api/projects/status/4pu2xnnrikol2gsf/branch/master?svg=true)](https://ci.appveyor.com/project/golang/dep/branch/master)
 
-Dep is a prototype dependency management tool.
+Dep is a prototype dependency management tool. It requires Go 1.7 or newer to compile.
 
 ### Dep is NOT an official tool. It is not (yet) blessed by the Go team.