Add testing instructions

[popzip]

Add testing instructions to contributing.md, primarily for external contributors. Will also be a good basis for internal team members to know pre-requisite for submitting pull request.

See simple example in the Roxy repo.

Include any Samplestack or product-specific guidelines to contributors

Related topics - work with @dmcassel: Confirm internal process for reviewing contributions from external contributors before accepting pull requests Who maintains '[email protected]'? (CLA sent here) Where is list of 'approved contributors' maintained? (to see if user has submitted CLA before)