add content to Continuous integration section

[jeffpaul]

Is your enhancement related to a problem? Please describe.
Let's add details to this section about adding CI for unit tests, WP Acceptance tests, and coding standards.

Describe the solution you'd like

• add details for adding unit tests
• add details for setting up CI to run unit tests
• add details for adding WP Acceptance tests
• add details for setting up CI to run WPA
• add details for coding standards (WPCS core? VIP? 10up ruleset?)
• add details for setting up CI to run coding standards
• remove hidden <!-- @todo: add callout for unit tests, WP Acceptance tests, coding standards -->

Designs
n/a

Describe alternatives you've considered
n/a

Additional context
n/a