Add async chat options to README

I've created two chat locations for users and potential developers of versioneer to chat in.

One is on Zulip:
* https://versioneer.zulipchat.com

The other is on Matrix/Element:
* https://matrix.to/#/%23python-versioneer:gitter.im

This should be added to our README and related documentation to be easier to find, but wanted to share it in an issue right now.

cc: @effigies 