Add a way to get notifications of updates

## Reporters would like to know every time a new card is added

As a reporter, I would like to get a notification every time a new company/app is added, so I can keep up with which states and countries are launching new apps.

## Background

We've got a lot of attention on the LFPH landscape right now, especially from reporters who want the latest and greatest info on who has an exposure notification app launched right now. If we could have a way to send them notifications whenever there's a change it helps LFPH remain top-of-mind for them when they're writing articles and means we're providing something valuable to them, which helps promote the work we're doing generally and makes us seem more legit.

### Requirements
* There is some method to subscribe to the landscape
* The subscription creates a notification every time a new card is added or removed
* The notification is probably an email to the reporter who subscribed (but could also be an RSS feed?)
* The notification itself has a link to the new card and (at least) a summary of what was added: name, URL, GitHub repo