appdata: add releases tag

Author: cosimoc

build/linux/appdata.xml

@@ -30,6 +30,13 @@
 
  <content_rating type="oars-1.1" />
 
+ <releases>
+ <release date="2018-07-26" version="3.4" />
+ <release date="2018-07-22" version="3.3.7.2" />
+ <release date="2018-07-01" version="3.3.7.1" />
+ <release date="2018-03-13" version="3.3.7" />
+ </releases>
+
  <url type="homepage">http://www.processing.org/</url>
  <url type="help">https://processing.org/reference/</url>
  <url type="bugtracker">https://github.com/processing/processing/issues?q=is%3Aopen</url>