added --extent option to man page

git-svn-id: http://svn.osgeo.org/qgis/trunk@4999 c8812cc2-4d05-0410-92ff-de0c093fc19c

Author: wonder

qgis.man

@@ -55,6 +55,9 @@ Load the specified
 project file. The layers specified in the project file are loaded, the layers
 are symbolized, and the view extent is restored.
 .TP
+.B \--extent xmin,ymin,xmax,ymax
+Set initial map extent by passing coordinates of that rectangle.
+.TP
 .B \--help 
 .br 
 Display brief usage help.