layout: add a note about preloading

This should work from Firefox 78 once preloading is enabled again.
See:

 - https://bugzilla.mozilla.org/show_bug.cgi?id=1623953
 - https://bugzilla.mozilla.org/show_bug.cgi?id=1626997

layout/base.j2

@@ -39,7 +39,7 @@
 {% endif %}
   <link rel="alternate" href="{{ content_url( resource.meta.language + '/blog/atom.xml') }}" type="application/atom+xml" title="ATOM feed">
 {% for font in "merriweather merriweather-italic iosevka-custom-regular".split() %}
-  <link rel="preload" href="{{ media_url('fonts/') }}{{ font }}.woff2" as="font" type="font/woff2">
+  <link rel="preload" href="{{ media_url('fonts/{}.woff2'.format(font)) }}" as="font" type="font/woff2">
 {% endfor %}
   <link rel="stylesheet" href="{{ media_url('css/luffy.css') }}">
 {% for css in resource.meta.css %}