Code Guidelines Updated

@jameskiesel @hannahsalmon @richarddbartlett @mixmix Could you please have a read and give me any feedback on this WIP: https://github.com/loomio/loomio/wiki/Code-Review-Checklist

Rob I love it. I made some little notes:

• Is this the right name for that document? The document rules but the name makes me want to ignore it :)
• Do you have 1 or 2 great 'intro to BEM' articles you can link in there?
• At a later date it would be cool to add an exapanded section about mixins.scss and utilities.scss

This is so great.

hey, looks good. I just skimmed it. surface observations :
- link up mentions of folders to actual locations
- consider a linked contents at the top?
- is this a guide or a checklist? I think there should be a literal bullet-point checklist of questions with supplementary clarifications below