Describe WHY we do things the way we do

# 💡 Summary #

We do a decent job in this guide of describing the standards and practices we choose, but not such a great job of explaining _why_ we chose them. We should make an effort to do so.

## Motivation and context ##

In a dev team meeting today I mentioned that:
- Historically we do a terrible job of getting new folks up to speed. If someone is a strong self-starter then they usually do OK, but our current process _strongly_ selects for self-starters.
- We have a difficult time getting contributors to read and follow the documentation in this repo.

It was mentioned that adding more explanation as to _why_ we do things in the way we do might help with both issues.