add brotli bug and workaround to docs

for #878

templates/about.html

@@ -583,6 +583,8 @@ <h3 id="listen">Pulling back responses</h3>
 <a href="https://github.com/snarfed/bridgy/issues/483">also looks for links
 in your profile's description field</a>.
 </p>
+
+<p>Also, if your web server has <a href="https://brotli.org/">Brotli compression</a> enabled, Bridgy won't work with it due to <a href="https://issuetracker.google.com/issues/112277350">this long-standing App Engine bug</a>. <a href="https://github.com/snarfed/bridgy/issues/878">More background.</a> Until this bug is fixed, we recommend disabling Brotli on your web server as a workaround.</p>
 </li>
 
 <li id="order" class="question">Why are the Bridgy comments on my site out of