add upstream URL to READMEs

Change-Id: I9cde5f94c81c59ff99b8f5af9765b94b6f57184a

external/libabw/README

 AbiWord file word processor format import library from
 
-I couldn't find an upstream for this library - although I did only try a primitive Google search.
+[https://wiki.documentfoundation.org/DLP/Libraries/libabw]

external/libcdr/README

 Library parsing the Corel cdr documents.
 
-I couldn't find an upstream for this library - although I did only try a primitive Google search.
+[https://wiki.documentfoundation.org/DLP/Libraries/libcdr]

external/libebook/README

 Library for import of various non-HTML reflowable e-book formats.
+
+[https://sourceforge.net/projects/libebook/]

external/libetonyek/README

 Library for import of Apple Keynote presentations.
+
+[https://wiki.documentfoundation.org/DLP/Libraries/libetonyek]

external/libfreehand/README

 Library for import of FreeHand drawings.
+
+[https://wiki.documentfoundation.org/DLP/Libraries/libfreehand]

external/libmspub/README

 Library parsing the Microsoft Publisher documents.
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+[https://wiki.documentfoundation.org/DLP/Libraries/libmspub]

external/libpagemaker/README

 Library parsing Adobe PageMaker documents.
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+[https://wiki.documentfoundation.org/DLP/Libraries/libpagemaker]

external/libvisio/README

 Libvisio is library providing ability to interpret and import visio diagrams into various applications.
 
-See also:
 [https://wiki.documentfoundation.org/DLP/Libraries/libvisio]

external/libzmf/README

 libzmf is a library and a set of tools for reading and converting
 Zoner Draw and Zebra file formats.
 
-libzmf requires boost and librevenge to build.
-
-The library is available under MPL 2.0+ license.
+[https://wiki.documentfoundation.org/DLP/Libraries/libzmf]