Document using ng test --browsers ChromeHeadless for remote test running

[NicMcPhee]

If you want use ssh to connect to a lab machine and run the tests remotely just to make sure a branch works in the lab, you need to run the Angular unit tests (ng test) using a headless browser, i.e., one that won't try to display a window and show you the animation of the tests and their results.

This can be easily accomplished by specifying --browsers ChromeHeadless as an argument to ng test, i.e.,

ng test --browsers ChromeHeadless --no-watch

I added --no-watch above because I'm guessing that will be how it will normally be run, but the --no-watch isn't necessary and that should be made clear somewhere/somehow.

This should be added to the list of ng commands in the README. Addressing this issue is literally just adding a line or two of text (my write-up here is far longer), so it's an easy one for someone to grab.