Proposed change to set.mm repo README

[David A. Wheeler]

The README.md of the "set.mm" repo said almost nothing.

So I significantly extended it in this proposed change:
https://github.com/metamath/set.mm/pull/2373/files

Basically, I tried to provide helpful information so that if someone
started at *that* page, they'd eventually find hopefully-useful information.

I tried to briefly document what I *think* we mostly agreed on as
the process for merging changes. If I got it wrong, please discuss.
I think it's important to document *some* rule, so that everyone knows
what the rules are.

I also put in a brief memorium for Norm (at the end).

--- David A. Wheeler

[Alexander van der Vekens]

I generally agree with David's approch. I have only some minor remarks (see review comments in the PR).

Especially the memorium for Norm is a good idea - it is worth to put it in a prominent place.