clarify when hmac empty string errors occur

Author: jonmchan

README.md

@@ -77,7 +77,7 @@ puts decoded_token
 * HS512 - HMAC using SHA-512 hash algorithm
 
 ```ruby
-# The secret must be a string. With OpenSSL 3.0, JWT::DecodeError will be raised if it isn't provided.
+# The secret must be a string. With OpenSSL 3.0/openssl gem `<3.0.1`, JWT::DecodeError will be raised if it isn't provided.
 hmac_secret = 'my$ecretK3y'
 
 token = JWT.encode payload, hmac_secret, 'HS256'