[Signup Flow] More clarity around what the username / email address is used for

When signing up, I didn't know what my email address was for: Keybase sending me mail, password recovery, etc. or other people sending me mail. Should I use an address I expect to be sending mail from? Would there be an opportunity to validate multiple email addresses?
It also is not clear whether the email address I use to sign up will be publicly displayed on my profile.

I think a blurb on the signup page indicating that the email address is only for communication between Keybase and me would be useful, along with a popup with examples of the types of email Keybase might send.

Similar concerns surround the username. It should be clear that the username is your publicly visible name and the URL by which your account will be visible.

Start at the signup page and locate the username and email-address fields. Review how profile visibility and account URLs are explained, then add guidance covering email communication, possible message types, public visibility, and the username’s public role. Done means a new signup user can answer these questions from the page itself.