added a "first steps" part in GEOM documentation with a first entry about differences between fuse compound and partition

doc/salome/gui/GEOM/input/first_steps_guide.doc

@@ -0,0 +1,12 @@
+/*!
+
+\page first_steps_guide First steps in GEOM
+
+You can find here some useful information to start working with GEOM
+
+<ul>
+<li>\subpage partition_explanation "Partition, compounds and boolean operations" </li>
+</ul>
+
+
+*/

doc/salome/gui/GEOM/input/index.doc

@@ -4,6 +4,9 @@
 
 \image html geomscreen.png
 
+If you are a new user some general information about GEOM usage are given here:
+\subpage first_steps_guide
+
 \b Geometry module of SALOME is destined for: 
 - \subpage import_export_geom_obj_page "import and export of geometrical models"
   in IGES, BREP and STEP formats;

doc/salome/gui/GEOM/input/partition_explanation.doc

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+/*!\page partition_explanation Fuse, Partition and Compound usage
+
+It is frequently asked about the difference between the above mentioned operations. It's indeed simple. Lets take the example of a cylinder and a box that you want to join together.
+
+\section Fuse
+
+The \b fuse operation will make a <b>single solid</b> from the two given solids :
+
+\image html fuse.png
+
+\section Partition
+
+The \b partition operation will do basically the same but it will <b>keep a face at the frontier between the two solids</b> (in brown on the picture below). 
+This face is shared by the two solids, which means that this face is present only one time in the resulting shape and is a sub-shape of the box and a sub-shape of the cylinder.
+
+This operation allows you to identify different areas in a shape (e.g. different materials) and to ensure a conformal mesh when meshing it later. Indeed the face at the frontier is meshed only one time. 
+
+\image html partition.png
+
+\section Compound
+
+When you build a \b compound by using the build -> compound operation you just make <b>an object that contains the two separate solids</b> like in a "bag".
+The two solids remain unconnected. The compound is just a set of shapes, no more.
+
+\image html compound2.png
+
+*/