Roadmap for release 0.5

[JanMarvin]

This will be release 0.5 and should be the fifth released to CRAN.

I'm currently planing with a release at the end of the month. There are a couple of open issues and pull requests in milestone 0.5, but for the moment I don't really work on openxlsx2 and a few of the current changes are addressing issues reported my users that should be pushed to CRAN.

open pull requests to merge

• N/A

open pull requests to modify

• 473 na strings #474
• 477 file ext #485 (required after all?)

issues to work on pre release

• update wb_add_chart_xml description #483