Mark this repository as an archive

Someone who has access to [this repository's settings](https://github.com/openknowledge-archive/activityapi/settings) should go there and mark "**Archive this repository**", so that we don't get new issues or pull requests on this project which is *de facto* archived.

![github-archive](https://user-images.githubusercontent.com/1058414/146267911-be9d4928-5694-4a14-a0b7-efdefad7585d.png)