[Docs] add explicit info about customization of homepage (#8694)

* first draft

* Update docs/content/doc/advanced/customizing-gitea.en-us.md

Co-Authored-By: zeripath <art27@cantab.net>

* add notice to restart

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -72,6 +72,12 @@ matching source.
 Any statement contained inside `{{` and `}}` are Gitea's template syntax and
 shouldn't be touched without fully understanding these components.
 
+### Customizing startpage / homepage
+
+Copy [`home.tmpl`](https://github.com/go-gitea/gitea/blob/master/templates/home.tmpl) for your version of Gitea from `templates` to `custom/templates`.
+Edit as you wish.
+Dont forget to restart your gitea to apply the changes.
+
 ### Adding links and tabs
 
 If all you want is to add extra links to the top navigation bar, or extra tabs to the repository view, you can put them in `extra_links.tmpl` and `extra_tabs.tmpl` inside your `custom/templates/custom/` directory.