Added Processing section.

doc/src/Processing.rst

+.. File Processing.rst
+
+.. include:: global.rst
+
+.. _processing:
+
+Processing
+==========
+
+The complete analysis of an image is fully automated (:numref:`fig_sexlayout`). Two passes are made through the data.
+During the first pass, a `model of the sky background <background_mode>`_ is built, and several global statistics are computed.
+During the second pass, image pixels are background-subtracted, filtered and segmented on-the-fly.
+Detections are then deblended, pruned (“CLEANed”), and enter the measurement phase.
+Finally, the measured quantities are written to the output catalog, after cross-matching with an optional input list.
+
+.. _fig_sexlayout:
+
+.. figure:: figures/sexlayout.*
+   :align: center
+
+   Layout of the main |SExtractor| procedures. *Dashed arrows* represent
+   optional inputs. 
+
+The following sections describe each of these operations in more detail.
+
+.. toctree::
+
+   Background
+   Weighting
+   Flagging
+

doc/src/index.rst

@@ -17,9 +17,7 @@ Contents
    License
    Installing
    Using
-   Background
-   Weighting
-   Flagging
+   Processing
    Measurements
    references