Add status message and archive repo

[nanxstats]

Today, I was asked if gsdmvn is still needed for installing gsDesign2, while in reality, the gsdmvn functionalities have been merged into gsDesign2 for more than 6 months.

Therefore, we should consider archiving this repo following the GitHub documentation.

Before clicking the archiving button, please send a pull request to add this section to the README:

## Status [![Lifecycle: superseded](https://img.shields.io/badge/lifecycle-superseded-orange.svg)](https://www.tidyverse.org/lifecycle/#superseded)

gsdmvn is superseded: the functionality was included directly in the gsDesign2 package.
We recommend using [gsDesign2](https://github.com/Merck/gsDesign2) instead.

[fb-elong]

Are you able to hit the archive button after #48 merge?

@LittleBeannie, could you take a look the issue log and see if any issues need to be moved into gsDesign2?