create place to put Slack archives

We had discussed setting up automatic Slack archiving (discussed here: https://thegooddocsproject.groups.io/g/main/topic/34123164#177) but our channel is pretty light!

I've set myself a reminder to create an archive on the 10th of each month. I'm making this pull request so we have a place to put the archives.

I'll add a note in the Plumbing and Maintenance wiki page about this. :)