WIP style guide

First pass at a style guide. Most of this *should* be common sense. Yet, as everything listed here frequently comes up in code reviews, there is arguably a need to make common sense a little more common.

I expect there will be some debate over what is a juju convention/style vs someone's opinion. As such, I provide this doc only as a starting point and I am not attached to any given convention in there. The benefits of agreeing on and clarifying our conventions upfront are: easier 'on-ramping' for newbies and quicker reviews.

I will start a juju-dev thread for this PR, please use it for debating what should/shouldn't be in the style guide (or even if you disagree that we need a guide).
Because it's contentious? I think named returns need to be addressed. If this guideline is disagreed with, leaving it in begs the question: how do we correctly use named return values? I for one would benefit from that debate being settled.

As to lines, each section ends with two new lines before next heading, so I'll leave 99, 100. I'll remove 97 though.