Add GoDoc badge and go get instructions (#53)

diff --git a/README.md b/README.md
index dc68008..d6e214e 100644
--- a/README.md
+++ b/README.md
@@ -3,12 +3,19 @@
 
 [![Build Status](https://travis-ci.org/googleapis/gax-go.svg?branch=master)](https://travis-ci.org/googleapis/gax-go)
 [![Code Coverage](https://img.shields.io/codecov/c/github/googleapis/gax-go.svg)](https://codecov.io/github/googleapis/gax-go)
+[![GoDoc](https://godoc.org/github.com/googleapis/gax-go?status.svg)](https://godoc.org/github.com/googleapis/gax-go)
 
 Google API Extensions for Go (gax-go) is a set of modules which aids the
 development of APIs for clients and servers based on `gRPC` and Google API
 conventions.
 
-Application code will rarely need to use this library directly,
+To install the API extensions, use:
+
+```
+go get -u github.com/googleapis/gax-go
+```
+
+**Note:** Application code will rarely need to use this library directly,
 but the code generated automatically from API definition files can use it
 to simplify code generation and to provide more convenient and idiomatic API surface.