Add godoc badge to README.md

Also put all 3 badges on the same line.

README.md

@@ -1,8 +1,8 @@
 # cgroups
 
 [![Build Status](https://travis-ci.org/containerd/cgroups.svg?branch=master)](https://travis-ci.org/containerd/cgroups)
-
 [![codecov](https://codecov.io/gh/containerd/cgroups/branch/master/graph/badge.svg)](https://codecov.io/gh/containerd/cgroups)
+[![GoDoc](https://godoc.org/github.com/containerd/cgroups?status.svg)](https://godoc.org/github.com/containerd/cgroups)
 
 Go package for creating, managing, inspecting, and destroying cgroups.
 The resources format for settings on the cgroup uses the OCI runtime-spec found