Explain in invite email what keybase.io is

Now email explains only that you can accept invitation, fill your profile and gives no clue of what service is actually about.

A picture of the rat in the end of email doesn't make it any trustworthy either :)

Locate the invite-email template or entry point and read the current invitation wording. Done means the email clearly explains what keybase.io is and why the recipient should trust the invitation, with the updated message reviewed for clarity.

Written by the indexing model from the issue text.