Document our engineering processes

[larrysalibra]

This pull request takes the draft of the engineering handbook that was previously circulated on paper and adds additional sections documenting how we use Github sprint boards, review pull requests and other processes.

Most of this is from my impression of how we're currently working together. This isn't to say this is a list of best practices but documentation of our starting point from which we can iterate on and improve.

Would love everyone's feedback.

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[larrysalibra]

Thanks for the comments and feedback everyone. Think I've addressed everyone's comments - ready for another review!

[yknl]

Thanks for putting this together @larrysalibra, everything looks good to me.

[jcnelson]

Looks good to me!

[kantai]

Looks good @larrysalibra !