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

Change-Id: I961972a89cfb5ae5aa71c6b8ba034d8b23b43876
Reviewed-on: https://go-review.googlesource.com/c/sys/+/275888
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/README.md b/README.md
index ef6c9e5..3b7e728 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # sys
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/sys.svg)](https://pkg.go.dev/golang.org/x/sys)
+
 This repository holds supplemental Go packages for low-level interactions with
 the operating system.