add explanation of expected format for vector layers imported as trai…

…ning areas

gui/wxpython/iclass/g.gui.iclass.html

@@ -35,7 +35,6 @@ <h2>DESCRIPTION</h2>
   <li>write signature file</li>
   <li>import vector map</li>
   <li>export vector map with attribute table</li>
-
 </ul>
 
 <center>
@@ -62,6 +61,14 @@ <h2>DESCRIPTION</h2>
 By doing this, the user can see how much of the image
 is likely to be put into the class associated with the signature.
 
+<p><em>wxIClass</em> can also import training areas defined in a vector layer. In that case the program expects the vector layer to have the following columns defined:
+<ul>
+    <li>cat: category value</li>
+    <li>class: a string with the class name</li>
+    <li>color: a color defined using a with format "RRR:GGG:BBB"</li>
+</ul>
+</p>
+
 <p>
 The spectral signatures are composed of region means and covariance matrices.
 These region means and covariance matrices are used in