Added Zapier integration

I wanted to empower users with what they wanted to do once a user signed up. If they wanted to trigger an email to the user they could do that or if they wanted to get notified on slack or export data into spreadsheets/Airtable Zapier could enable all that. I plan to build native integrations for others in the future but Zapier helps close the gap for those till then.

The integration can be found here