[Epic] Push Notifications

As a person using the @dwyl app to track my time spent on activities/tasks,
I would like to receive a notification informing me that:
+ [ ] The timer on my current activity has _exceeded_ the original estimate

_Before_ we implement Chrome (Browser) Push Notifications,
we need to _document_ how it's done. (_Document Driven Development!_)
GOTO: https://github.com/dwyl/learn-push-notifications/issues/1

When we start work on "Team Mode" https://github.com/dwyl/time/issues/219 additional notifications need to be sent:
+ [ ] A member of your team/community has requested your help on a task.
+ [ ] A Task assigned to someone else has been marked as "ready for review"