Add Sendgrid Plugin

Without email there is no account recovery option.

On my first install I lost track of the secure password I had generated. It meant I had to destroy the heroku app and begin again. Not a bad deal when getting started. If I had to do that today I would be unhappy. This again is free functionality which I consider critical to the viability of Wordpress on Heroku.

There are a few more instructions that need to be added to the wiki. Basically, activate the sendgrid plugin and then use the `heroku config` command to get the username and password from the environment variables and enter them into the WP config screen for the Sendgrid plugin.

This is what I used for my WP-Heroku install. Happy to hear other options/solutions etc.