Minor tidyups to the README

Make it a little easier for someone who has no idea what GovDelivery is
to understand this app, by rewording a bit and linking to
www.govdelivery.com/
Merge pull request #38 from alphagov/readme-tweaks

Minor tidyups to the README
Log count of subscriber lists emails are sent to

Without this logging, it's hard to tell whether the service has been
sending anything to GovDelivery.
Merge pull request #39 from alphagov/log-count-of-lists-sent-to

Log count of subscriber lists emails are sent to
Correct notifications controller test file name
Merge pull request #40 from alphagov/correct-notification-worker-spec-filename

Correct notifications controller test file name
Log the list of delivery ids an email is sent to

I can't see the emails the api claims to have sent in the govdelivery admin; hopefully if we log the delivery ids I'll have more chance of finding them.
Merge pull request #41 from alphagov/log-list-of-delivery-ids

Log the list of delivery ids an email is sent to
Make it explicit that secrets are overwritten
Merge pull request #42 from alphagov/secrets-yml-is-overwritten

Make it explicit that secrets are overwritten