Add a Github notification section

[ChihChengLiang]

As a developer subscribes to more repositories, Github web-based notifications make it hard to focus on the tasks that matter.

Adding a section of setting up and optimizing email notification in the manual would greatly improve the productivity of an onboarding snake charmer.

The proposed setup steps are:

1. Turn on the email notifications.
2. Route notifications from the Github organization to the work email address.
3. Setup labels for author, mention, and review_requested by filtering Cc address.
4. Optionally, setup labels for merged and closed by filtering the keywords.

[cburgdorf]

3. Setup labels for author, mention, and review_requested by filtering

That's a great idea!