[enh] add js to install on firefox as search engine

searx/templates/about.html

 <h2>FAQ</h2>
 <h3>Trust</h3>
 <p>It's ok if you don't trust us regarding the logs, <a href="https://github.com/asciimoo/searx">take the code</a> and run it yourself for your friends! decentralize!</p>
+<h3>Firefox engines plz?</h3>
+<p><a href="#" onclick="window.external.AddSearchProvider(window.location.protocol + '//' + window.location.host + '/opensearch.xml')">
+	Install</a> searx as a search engine on any Firefox!</p>
 
 
 </div>