Set up Mailchimp automated emails

Set up Mailchimp automated emails for new signs up through Meetup. Update Meetup Welcome message to add:

"1. Sign up for our email list: eepurl.com/gTNkXb . We will send a short sequence of on-boarding emails over the next week to help you learn about the organization and prepare for a meeting. You will also get quarterly update emails and alerts for special events."