Join: add specific branding requirements

We need to be specific in order to streamline things for last minute
joiners.

The requirements are adapted from what the Sentry design team asked for:
#115 (comment)

src/pages/join.astro

@@ -224,10 +224,29 @@ import Layout from "../layouts/Layout.astro";
             <span class="circled">5</span>
           </div>
           <div class="step">
-            <h3><span class="inline-number">5. </span>Include links to your high-resolution logo,</h3>
-            and optionally any other branding materials, in the pull request so that we can promote you! For those
-            pledging before September 24, this means you'll have a chance to be included in our major outdoor advertising
-            campaign, subject to available space and depending on how many pledges we get.
+            <h3><span class="inline-number">5. </span>Include your branding materials in the PR.</h3>
+
+            <p
+              >We need at least:
+
+              <ol>
+                <li>a glyph/icon in a square area, and</li>
+                <li>a full name/wordmark in a wide rectangle,</li>
+              </ol>
+
+              or your nearest equivalents. For both versions, you should either include a high-resolution (&gt; 800px
+              wide) raster file with a transparent background (e.g. PNG), or preferably a vector file (e.g. SVG).</p
+            >
+
+            <p
+              >Expect the logos to be on a dark background, so include a light version for your logo, or expect us to
+              make it all white.</p
+            >
+
+            <p
+              >For those pledging before October 8, you'll still have a chance to be included in our major outdoor
+              advertising campaign, subject to available space and depending on how many pledges we get.</p
+            >
           </div>
         </div>