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).
I don't agree with this.  Either form is valid.  I tend to only use var when the := form is especially unwieldy, like when making an interface value, because var err error is a lot more clear than err := error(nil)  ... but the two examples you give here are both equally easy to read, so there's no need to specify one over the other.  I'd probably prefer the non-var version because I think var is generally just more noise.