Add basic appdata file.

Gearboy.appdata.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Copyright 2017 Daniel Moerner <dmoerner@gmail.com> -->
+<component type="desktop">
+  <id>Gearboy.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPLv3+</project_license>
+  <name>Gearboy</name>
+  <summary>Game Boy and Game Boy Color Emulator</summary>
+  <description>
+    <p>
+      Gearboy is a Nintendo Game Boy and Game Boy Color emulator written in
+      C++. The emulator prioritizes readable, simple source code and high
+      compatibility across a variety of platforms and architectures. Gearboy
+      features highly accurate CPU emulation, instruction, and memory
+      timing. It also includes memory bank controllers, accurate LCD
+      controller emulation, sound emulation, and compressed ROM support.
+    </p>
+  </description>
+  <url type="homepage">https://github.com/drhelius/Gearboy</url>
+  <update_contact>dmoerner@gmail.com</update_contact>
+</component>