Clarify support status of OpenSlide on openslide.org

[adamu]

• The last mailing list announcement was November 2017
• The last commit was in May 2018
• None of the pull requests have received any interaction from a member of this project since around that time.

[agoode]

You can check the openslide-users mailing list, but this is currently the most accurate summary of the situation: https://lists.andrew.cmu.edu/pipermail/openslide-users/2019-March/001522.html

I will leave this issue open as documentation.

[Racater]

any suggested replacement?

[bgilbert]

OpenSlide 4.0.0 has been released and maintenance is ongoing, so I'll close this out.