Make a minor update to the docs

doc.go

@@ -2,6 +2,9 @@
 // spreadsheets stored in the XLSX format used in recent versions of
 // Microsoft's Excel spreadsheet.
 //
+// Additionally, xlsx has started to grow some XLSX authoring
+// capabilities too.
+//
 // For a concise example of how to use this library why not check out
 // the source for xlsx2csv here: https://github.com/tealeg/xlsx2csv