Add appcast.xml for ContainerUI update feed

- Created appcast.xml to support automatic updates via Sparkle for ContainerUI.
- Included initial release details for version 0.1, featuring container management and XPC service architecture.
- Added metadata such as versioning, publication date, and download link for the release.

Author: lcandy2

update-feed/appcast.xml

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="utf-8"?>
+<rss version="2.0" xmlns:sparkle="http://www.andymatuschak.org/xml-namespaces/sparkle" xmlns:dc="http://purl.org/dc/elements/1.1/">
+    <channel>
+        <title>ContainerUI Updates</title>
+        <link>https://github.com/YOUR_USERNAME/container-ui</link>
+        <description>Updates for ContainerUI - macOS Container Management App</description>
+        <language>en</language>
+        
+        <item>
+            <title>Version 0.1</title>
+            <link>https://github.com/YOUR_USERNAME/container-ui</link>
+            <sparkle:version>2</sparkle:version>
+            <sparkle:shortVersionString>0.1</sparkle:shortVersionString>
+            <description><![CDATA[
+                <h2>ContainerUI 0.1</h2>
+                <ul>
+                    <li>Initial release</li>
+                    <li>Container management with native macOS interface</li>
+                    <li>XPC service architecture for sandbox compliance</li>
+                    <li>Automatic update support via Sparkle</li>
+                </ul>
+            ]]></description>
+            <pubDate>Thu, 19 Jun 2025 12:00:00 +0000</pubDate>
+            <enclosure url="https://github.com/YOUR_USERNAME/container-ui/releases/download/v0.1/ContainerUI-0.1.zip"
+                       length="0"
+                       type="application/octet-stream"
+                       sparkle:edSignature="SIGNATURE_PLACEHOLDER" />
+            <sparkle:minimumSystemVersion>15.0</sparkle:minimumSystemVersion>
+        </item>
+        
+    </channel>
+</rss>