Add AppData file

[Alexander-Wilms]

This file would need to be installed to /app/share/appdata/cutemarked.appdata.xml on Linux, so CuteMarkEd shows up in Gnome Software and KDE Discover.

The metadata_data license should actually be one of

• FSFAP
• CC0-1.0
• CC-BY-3.0
• CC-BY-SA-3.0
• GFDL-1.3
• MIT

so that the appdata can be incorporated into the distribution metadata.

<?xml version="1.0" encoding="UTF-8"?>
<component type="desktop-application">
	<id>cutemarked.desktop</id>
	<name>CuteMarkEd</name>
	<project_license>GPL-2.0+</project_license>
	<metadata_license>GPL-2.0+</metadata_license>
	<summary>Qt Markdown Editor</summary>
	<developer_name>Christian Loose</developer_name>
	<description>
		<p>
		A Qt-based, free and open source Markdown editor with live HTML preview, math expressions, code and markdown syntax highlighting.
		</p>
		<p>Features:</p>
		<ul>
			<li>Multiple styles</li>
			<li>Live HTML Preview</li>
			<li>HTML and PDF Export</li>
			<li>Document statistics</li>
			<li>Code Syntax Highlighting</li>
			<li>Spell Checking</li>
		​</ul>
	</description>
	<screenshots>
		<screenshot type="default">
		<image type="source">https://cloose.github.io/CuteMarkEd/images/screenshot_06.png</image>
		</screenshot>
	</screenshots>
	<url type="homepage">https://cloose.github.io/CuteMarkEd/</url>
	<url type="bugtracker">https://github.com/cloose/CuteMarkEd/issues</url>
	<url type="help">https://github.com/cloose/CuteMarkEd/blob/develop/README.md</url>
</component>