add readme for svl.

Change-Id: I20addd47e79bec6a260baf391fbb495da6fdaf57

svl/README

+Contains non-graphical helper code for office applications.
+
+In particular the SfxItemSet is a property-bag like container that
+stores arbitrary sets of properties for everything from text run
+formats, to Chart regression line properties.
+
+There are lots of other useful helpers in here for various office
+tasks; much of this code was originally moved from svx/sfx2.
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