publishers.html: add guidance on URL directory paths

extension_explorer/templates/publishers.html

@@ -175,7 +175,7 @@ <h3>
         </h3>
 
         <p>
-          {{ _("You will need to find somewhere to host your extension online at a stable URL. Creating a GitHub repository for your extension is a good option because it offers version control and issue tracking.") }}
+          {{ _("You will need to find somewhere to host your extension online at a stable URL. It must be possible to access an extension's schema and codelist files by replacing extension.json in the extension's URL with a file's path, e.g. <code>release-schema.json</code> or <code>codelists/codelist.csv</code>. A permissible alternative is to use an API that behaves like a directory. Creating a GitHub repository for your extension is a good option because it offers version control, issue tracking and provides the appropriate file URLs.") }}
         </p>
 
         <p>