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

A badge linking to https://pkg.go.dev/golang.org/x/crypto is added.

Change-Id: Ifa07c0c04e3dd30503f196179f6883cc569e7908
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/275873
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>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

README.md

@@ -1,5 +1,7 @@
 # Go Cryptography
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/crypto.svg)](https://pkg.go.dev/golang.org/x/crypto)
+
 This repository holds supplementary Go cryptography libraries.
 
 ## Download/Install