Add more examples to readme

[stevector]

The readme should have a list of public repos that implement this orb.

Currently I'm using it in

• https://github.com/stevector/nerdologues-d8
• https://github.com/stevector/stevector-composer
• https://github.com/stevector/wordpress-orb-demo
• (WIP) Use Pantheon orb example-drops-8-composer#245