warning: Display warning when using GPG short ID

The use of GPG short ids is insecure is insecure.  It is trivial to
create a colliding short id with the use of inexpensive hardware.  As
GPG still uses short ids as their default behavior, it is important to
meet users where they're at and while not causing things to break, we
should inform and discourage users from this practice.

Affects #10

main.go

@@ -259,6 +259,7 @@ func checkGPG(file *File) (state SigState, err error) {
 		// short or long id. If it's not 8 or 16, it's not valid.
 		switch l {
 		case 8:
+			fmt.Println("WARNING: The use of short ids is NOT secure.  See https://evil32.com for more info.")
 			rid = signer.PrimaryKey.KeyIdShortString()
 		case 16:
 			rid = signer.PrimaryKey.KeyIdString()