Adding optional listing of former project maintainers and displaying them in README

[palmskog]

To give proper credit to our maintainers, I think it would be appropriate to store the coq-community maintainership history in meta.yml, regardless of whether it is displayed in README.md or not. Based on discussion in the Zulip, there are two convenient ways to do this in meta.yml, either as part of the maintainers list or as part of a new list.

First option example:

maintainers:
- name: Karl Palmskog
  nickname: palmskog
  current: true
- name: Hugo Herbelin
  nickname: herbelin

Second option example:

maintainers:
- name: Karl Palmskog
  nickname: palmskog

former_maintainers:
- name: Hugo Herbelin
  nickname: herbelin

Note that former_maintainers does not necessarily need to contain the nicknames.