Add a few keywords for easier finding in stores

This is supported by AppStream
https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-keywords
and can be used in the Microsoft Store.

org.kde.neochat.appdata.xml

@@ -200,6 +200,11 @@ to provide a convergent experience across multiple platforms.</p>
       <li xml:lang="x-test">xxLocation Events - MSC3488xx</li>
     </ul>
   </description>
+  <keywords>
+    <keyword>instant messaging</keyword>
+    <keyword>chat</keyword>
+    <keyword>matrix</keyword>
+  </keywords>
   <url type="homepage">https://apps.kde.org/neochat/</url>
   <url type="bugtracker">https://bugs.kde.org/buglist.cgi?component=General&amp;product=NeoChat</url>
   <categories>