add join link to win-open-data mailing list

Submitted a request to create win-open-data@maillist.ox.ac.uk for external users to subscribe to updates (15/07/21).

Logged: Support Call# 11111087 - List "win-open-data@maillist.ox.ac.uk" creation request from cassandra.gouldvanpraag@psych.ox.ac.uk

Also expect mailing list subscribe to be included in data availability statements for papers currently being prepared for submission.

When request has been approved update the following: