Update README.md

README.md

@@ -42,3 +42,14 @@ to remove the PageGroup.
 
 Ralf Ulrich
 May 2020
+
+# References
+
+The starting point to this project were:
+
+https://tex.stackexchange.com/questions/76273/multiple-pdfs-with-page-group-included-in-a-single-page-warning
+
+https://www.mrunix.de/forums/showthread.php?73546-pdfTeX-warning-pdflatex-exe-PDF-inclusion-multiple-pdfs-with-page-group-included-in
+
+There are actually options to edit PDF files based on "qdf" that will be more general -- which I did not need. Also QDF will change the files
+in a much more invasive way also yieldig significantly larger files in the end. 
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