README.md: add badge to pkg.go.dev

Change-Id: I48dc1611bd6e85697a64afb66aa6ae79a60adfce
Reviewed-on: https://go-review.googlesource.com/c/arch/+/275933
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

README.md

@@ -1,5 +1,7 @@
 # arch
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/arch.svg)](https://pkg.go.dev/golang.org/x/arch)
+
 This repository holds machine architecture information used by the Go toolchain.
 The parts needed in the main Go repository are copied in.