title: Generate site.xml from existing update site
body: <p>When I have an Eclipse update site project, a site.xml file is created which looks like:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;site&gt;
     &lt;feature url="features/org.eclipse.egit_1.1.0.201109151100-r.jar" id="org.eclipse.egit" version="1.1.0.201109151100-r"/&gt;
     &lt;feature url="features/org.eclipse.draw2d_3.7.1.v20110830-1143-4607w3122194102254.jar" id="org.eclipse.draw2d" version="3.7.1.v20110830-1143-4607w3122194102254"&gt;
        &lt;category name="test_cat1"/&gt;
     &lt;/feature&gt;
  &lt;category-def name="test_cat1" label="Test Catergory 1"&gt;
     &lt;description&gt;
     This be a test
     &lt;/description&gt;
  &lt;/category-def&gt;
  &lt;/site&gt;
  </code></pre>
  
  <p>But if I have an existing  update site (created by for example B3 aggregator), is there an easy way to generate this site.xml file?</p>