Update metainfo.xml to comply with Flatpak.

Author: bryce-carson

org.messerlab.slimgui.metainfo.xml

@@ -24,4 +24,8 @@
   <url type="bugtracker">https://github.com/MesserLab/SLiM/issues</url>
   <url type="homepage">https://messerlab.org/slim/</url>
   <developer_name>Ben Haller</developer_name>
+  <content_rating type="oars-1.0" />
+  <releases>
+    <release version="v4.0" date="2022-08-27" type="stable"/>
+</releases>
 </component>