Add GoDoc badge to README

Author: emersion

README.md

@@ -1,5 +1,7 @@
 # go-openpgp-wks
 
+[![GoDoc](https://godoc.org/github.com/emersion/go-openpgp-wks?status.svg)](https://godoc.org/github.com/emersion/go-openpgp-wks)
+
 A Go library for [OpenPGP Web Key Service](https://tools.ietf.org/html/draft-koch-openpgp-webkey-service-04).
 
 ## License