README: add pkg.go.dev tag

Change-Id: Ic931d978a5f08a7127a964304b7c5c94bc79b7e7
Reviewed-on: https://go-review.googlesource.com/c/exp/+/275301
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>

Author: Jay Conrod

README.md

@@ -1,5 +1,7 @@
 # exp
 
+[![PkgGoDev](https://pkg.go.dev/badge/golang.org/x/exp)](https://pkg.go.dev/golang.org/x/exp)
+
 This subrepository holds experimental and deprecated (in the `old`
 directory) packages.