update README to include PAGERDUTY_SOURCE_URL parameter

Add missing parameter documentation from the v1.2.0 version.

fix missing username when referencing docker repo

v1

update README to include PAGERDUTY_SOURCE_URL parameter (#9)

Add a Pull Request URL to notification

Include a Pull Request URL to the notification for further examination

Use static name for reference to docker container

v1.1.3

use static container name based on repo name

Improve self-reference syntax in notifier

v1.1.2

use double brackets syntax for notifier action

Fix name/description issue with CI action

New CI action was failing due to adding name+description. Removed.

Add CI to build, tag and publish to Docker Hub

Eliminate the need to manually build image and publish to DockerHub.
If forking, you need to set DOCKERHUB_USERNAME and DOCKERHUB_PASSWORD secrets in your repository