package.xml

<package version="1.1">
  <filegroup source="http://example.com/path/to/folder/with/files/" dest=".">
    <archive size="SIZE_IN_BYTES">THE_ZIP_NAME.zip</archive>
  </filegroup>
  <filegroup source="http://s3.amazonaws.com/MinecraftDownload/" dest="bin" verify="md5">
    <file size="738345">lwjgl.jar</file>
    <file size="227833">jinput.jar</file>
    <file size="138318">lwjgl_util.jar</file>
  </filegroup>
  <filegroup source="http://s3.amazonaws.com/MinecraftDownload/" dest="bin/natives" verify="md5">
    <archive size="491232" platform="windows">windows_natives.jar.lzma</archive>
    <archive size="551247" platform="linux">linux_natives.jar.lzma</archive>
    <archive size="262436" platform="mac_os_x">macosx_natives.jar.lzma</archive>
    <archive size="272470" platform="solaris">solaris_natives.jar.lzma</archive>
  </filegroup>
</package>

update.xml

<update>
  <latest>PICK_A_VERSION</latest>
  <packageurl>http://example.com/path/to/package.xml</packageurl>
</update>