* etc/emacs.appdata.xml: New file

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

The description is adapted from the Emacs homepage.

etc/ChangeLog

+2014-08-28  Glenn Morris  <rgm@gnu.org>
+
+	* emacs.appdata.xml: New file; description from Emacs's homepage.
+
 2014-07-12  Eli Zaretskii  <eliz@gnu.org>
 
 	* PROBLEMS: Mention the problem from bug #17950.

etc/emacs.appdata.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Copyright (C) 2014 Free Software Foundation, Inc. -->
+<application>
+ <id type="desktop">emacs.desktop</id>
+ <metadata_license>GFDL-1.3</metadata_license>
+ <!-- It is GFDL-1.3+, but appdata-validate complains... -->
+ <project_license>GPL-3.0+ and GFDL-1.3</project_license>
+ <name>GNU Emacs</name>
+ <summary>An extensible text editor</summary>
+ <description>
+  <p>
+   GNU Emacs is an extensible, customizable text editor - and more.
+   At its core is an interpreter for Emacs Lisp, a dialect of the Lisp
+   programming language with extensions to support text editing.
+  </p>
+  <p>The features of GNU Emacs include:</p>
+  <ul>
+   <li>Content-sensitive editing modes, including syntax coloring, for
+   a wide-range of file types</li>
+   <li>Complete built-in documentation, including a tutorial for new users</li>
+   <li>Full Unicode support for nearly all human languages and their scripts</li>
+   <li>Highly customizable, using Emacs Lisp code or a graphical interface</li>
+   <li>Includes a project planner, mail and news reader, debugger
+   interface, calendar, and more</li>
+  </ul>
+ </description>
+ <screenshots>
+  <screenshot type="default" width="632" height="354">http://www.gnu.org/software/emacs/images/appdata.png</screenshot>
+ </screenshots>
+ <url type="homepage">http://www.gnu.org/software/emacs</url>
+ <updatecontact>emacs-devel_at_gnu.org</updatecontact>
+ <project_group>GNU</project_group>
+</application>