add upgrade documentation #492

UPGRADE-3.0.md

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+# Migrate from 2.x to 3.0
+
+## Handling of empty rows
+
+* The handling of empty data in writers has changed. In 2.x an array was not added to the spreadsheet when ```empty($dataRow)``` evaluated to true.
+* In 3.0 a row is always written to the speadsheet. When the row does not contain any cells an empty row is created in the sheet.