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

Change-Id: Ib182f3257938ce7233ed68436bbfb28ddd1053f1
Reviewed-on: https://go-review.googlesource.com/c/build/+/275234
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Julie Qiu <julie@golang.org>
diff --git a/README.md b/README.md
index d798a7d..978a1ff 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # Go Build Tools
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/build.svg)](https://pkg.go.dev/golang.org/x/build)
+
 This subrepository holds the source for various packages and tools that support
 Go's build system and the development of the Go programming language.
 
@@ -49,7 +51,7 @@
   1. by creating VMs on Google Compute Engine (GCE) with custom images
   configured to fetch & run the buildlet on boot, listening on port 80 in a
   private network.
-  
+
   2. by running Linux containers (on either Google Kubernetes Engine
   or GCE with the Container-Optimized OS image), with the container
   images configured to fetch & run the buildlet on start, also