Merge pull request #476 from tealeg/update-readme-to-reflect-v2

Update the README to reflect the push for v2.0.0

README.org

@@ -12,6 +12,17 @@ by recent version of Microsoft Excel in Go programs.
 The support for writing XLSX files is currently extremely minimal.  It
 will expand slowly, but in the meantime patches are welcome!
 
+** A note about versions
+
+As of October 8th, 2019, I've branched off v1.x.x maintenance work
+from master.  The master branch will now be moving towards a new 2.0.0
+milestone that will include breaking changes in the API.  The scope of
+these changes will include, but perhaps not be limited to, the way
+`Col` elements and `DataValidation` works, as these aspects have been
+built around incorrect models of the underlying XLSX format.
+
+See the [milestone](https://github.com/tealeg/xlsx/milestone/5) for details.
+
 ** Full API docs
 The full API docs can be viewed using go's built in documentation
 tool, or online at [[http://godoc.org/github.com/tealeg/xlsx][godoc.org]].