Add AppData file

http://people.freedesktop.org/~hughsient/appdata/

<?xml version="1.0" encoding="UTF-8"?>
<application>
 <id type="desktop">Cable.desktop</id>
 <licence>CC0</licence>
 <name>Cable</name>
 <summary>IRC client</summary>
 <description>
  <p>
Cable is a fabulous IRC client for elementary OS, freely available in terms of free beer and free speech.
  </p>
 </description>
 <screenshots>
  <screenshot type="default" width="1065" height="625">http://imageshack.us/a/img823/3935/29oq.png</screenshot>
 </screenshots>
 <url type="homepage">https://launchpad.net/cable</url>
 <updatecontact>spautz [döt] julien [ät] gmail [döt]com </updatecontact>
 <project_group>elementary</project_group>
</application>