Instructions to install dashboards webapp service account

Summary:
The dashboards webapp now uses on a service account to auth against
Google Analytics' API as of http://phabricator.khanacademy.org/D6572
and these instructions are needed to set the account up.

Test Plan: I followed the steps in the comments.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D6573