add push some refs error instructions

index.html

@@ -55,6 +55,9 @@ <h2>Participation Guidelines</h2>
             <pre>docker run --rm -it vocdoni/zk-voceremony</pre>
             <p>Then, simply follow the on-screen instructions.</p>
             <p>You can follow the ceremony process <a href="https://github.com/vocdoni/zk-voceremony/tree/ceremony/vocdoni-zkcensus-ceremony" target="_blank">in its branch</a>.</p>
+
+	    <p>If you encounter the following error <i>[rejected] failed to push some refs....</i> during the final step, please start the process anew. 
+	    This indicates that someone else has contributed before you. To proceed, you need to update your information and contribute again.</p>
         </div>
         <img src="contribute-demo.gif" alt="demo" style="max-width: 1200px;border: 2px solid #ffffff78;margin-top: 20px;">
     </div>