Add godoc link to README.md.

README.md

@@ -5,6 +5,11 @@ Installation
 
 Run `go get -u github.com/huin/goupnp`.
 
+Documentation
+-------------
+
+[![GoDoc](https://godoc.org/github.com/huin/goupnp?status.svg)](https://godoc.org/github.com/huin/goupnp)
+
 Regenerating dcps generated source code:
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