Added a paragraph about running mkpasswd/mkgroup commands after first install.

docs/libvir.html

@@ -1670,6 +1670,14 @@ source on Windows:
     (either click on the desktop icon or look for Cygwin bash shell
     in the Start menu). </p>
 
+  <p> The very first time you start the Cygwin bash shell, you may
+    find you need to run the <code>mkpasswd</code> and <code>mkgroup</code>
+    commands in order to create <code>/etc/passwd</code> and
+    <code>/etc/group</code> files from Windows users.  If this
+    is needed then a message is printed in the shell.
+    Note that you need to do this as Windows Administrator. </p>
+</li>
+
 <li>
   <p>
     Check out