Update README for the v2 release

Author: ericchiang

README.md

@@ -2,7 +2,7 @@ This is not an officially supported Google product
 
 # A Go YubiKey PIV implementation
 
-[![GoDoc](https://godoc.org/github.com/go-piv/piv-go/piv?status.svg)](https://godoc.org/github.com/go-piv/piv-go/piv)
+[![Go Reference](https://pkg.go.dev/badge/github.com/go-piv/piv-go/v2/piv.svg)](https://pkg.go.dev/github.com/go-piv/piv-go/v2/piv)
 
 YubiKeys implement the PIV specification for managing smart card certificates.
 This applet is a simpler alternative to GPG for managing asymmetric keys on a
@@ -15,6 +15,13 @@ a wrapper for YubiKey's ykpiv.h C library. This package aims to provide:
 * Idiomatic Go APIs
 * Modern features such as PIN protected management keys
 
+V2 of this package was released in 2024 to support newer kinds of management
+keys, and is now the default branch for new features. The import path is:
+
+```
+import "github.com/go-piv/piv-go/v2/piv"
+```
+
 ## Examples
 
 * [Signing](#signing)