add official specification PDF

src/views/index.ejs

@@ -45,6 +45,7 @@ section div {
         <p>
             This API was developed for personal use, but modified and later released for public use.
             It is an unofficial application with the sole purpose of simplifying QR code generation for universal payment ordes (<a href="https://upn-qr.si">https://upn-qr.si</a>).
+            You can read the specification <a href="/NavodilaZaProgramerjeUPNQR.pdf">here</a>.
             The application does not keep any long-term logs.
         </p>
     </section>