Update filter README and add a new one.

See http://nabble.documentfoundation.org/About-Svgreader-td4107050.html
Thank you Christina for these information

Change-Id: I56c4f5fdeec91e7e35990bffcf19451baa541552

filter/README

@@ -9,3 +9,5 @@ and are loaded and used in a uniform fashion from code in
 svtools/source/filter/filter.cxx. Others have different API and are
 loaded from other places. For instance "icgm" has ImportCGM, and is
 loaded and used by sd/source/filter/cgm/sdcgmfilter.cxx (!).
+Svgreader is used for "File->Open" and then to choose the svg file.
+For "Insert->Picture->From File", see svgio/source/svgreader directory.

svgio/README

+It contains svgio/source/svgreader which is used for "Insert->Picture->From File".