README.md: add badge to pkg.go.dev
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 <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

golang/crypto,golang/crypto,golang/crypto