Add more info about mailing list to communications tools lecture

[shaunagm]

We talked about improving the mailing list section of the communications tools lecture (current version is here. Suggestions included:

• talking about the structure of the list (you reply to a single email address, but it reaches a whole list of people)
• how to deal with a busy list: checking the activity of the list, setting up filters
• one way to teach how mailing lists work could be to have the presenter create a mailing list for the group and have everyone at the event join it

To address this issue, add additional slide to the presentation as needed. Make sure to add this information to both presenter-led and self-guided versions. For the "make an issue" activity, we may want to brainstorm either here in the issue tracker or on osctc-planning the best way to go about it.