update libodfgen README a bit

Change-Id: I9d0a7e782b1bd5955cb524153b8a7bdea9e174e7
diff --git a/external/libodfgen/README b/external/libodfgen/README
index efeffac..01454e0 100644
--- a/external/libodfgen/README
+++ b/external/libodfgen/README
@@ -1 +1 @@
From [http://libwpg.sourceforge.net/]. Library to generate ODT and ODG documents from libwpd and libwpg API calls.
From [https://sourceforge.net/p/libwpd/wiki/libodfgen/]. Library to generate ODT and ODG documents from librevenge API calls.