Add standalone implementation

Add standalone implementation, giving a new choice to avoid using deprecated libgnome-keyring

README.md

@@ -83,10 +83,12 @@ For example, you could say `login:My_Very_Long_Login_Password`. (You may use `se
 
 <details>
   <summary>To use my naive secret file creation script</summary>
+
 ```
 gnome-keyring-yubikey-unlock/create_secret_file.sh /path/to/your_secret [Your GnuPG public key]
 # input your keyring_name:password
 ```
+
 </details>
 
 Then, add the following command to gnome-autostart. If you don't know how to do it, [read me](doc/how-to-gnome-autostart.md).