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

Change-Id: I60b0ccc17038d457c105a0727139eb36d61b4e12
Reviewed-on: https://go-review.googlesource.com/c/term/+/275936
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/README.md b/README.md
index e0f390c..d03d0ae 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # Go terminal/console support
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/term.svg)](https://pkg.go.dev/golang.org/x/term)
+
 This repository provides Go terminal and console support packages.
 
 ## Download/Install