Add Linux standard appdata.xml

org.beeref.BeeRef.appdata.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop">
+    <id>org.beeref.BeeRef</id>
+    <launchable type="desktop-id">org.beeref.BeeRef.desktop</launchable>
+    <name>BeeRef</name>
+    <developer_name>BeeRef</developer_name>
+    <summary>A Simple Reference Image Viewer</summary>
+    <metadata_license>CC0-1.0</metadata_license>
+    <project_license>GPL-3.0</project_license>
+    <url type="homepage">https://beeref.org/</url>
+
+    <description>
+        <p>BeeRef lets you quickly arrange your reference images and view them while you create. Its minimal interface is designed not to get in the way of your creative process.</p>
+    </description>
+
+    <screenshots>
+      <screenshot type="default">https://github.com/rbreu/beeref/blob/main/images/screenshot.png?raw=true</screenshot>
+    </screenshots>
+
+    <releases>
+        <release version="0.2.0" date="2021-09-06"></release>
+    </releases>
+
+    <content_rating type="oars-1.1" />
+</component>