README: Add reference links for formula information

README.md

@@ -122,3 +122,9 @@ The package contains a **qtxlsx.pri** file that allows you to integrate the comp
 * http://www.ozgrid.com/Excel/excel-custom-number-formats.htm
 * http://stackoverflow.com/questions/894805/excel-number-format-what-is-409
 * http://office.microsoft.com/en-001/excel-help/create-a-custom-number-format-HP010342372.aspx
+
+### Formula
+
+* http://msdn.microsoft.com/en-us/library/ff533995%28v=office.12%29.aspx
+* http://msdn.microsoft.com/en-us/library/dd906358%28v=office.12%29.aspx
+* http://homepages.ecs.vuw.ac.nz/~elvis/db/Excel.shtml