Move Trello guide into Playbook

A more recent version of this guide has been moved and edited in the
playbook:

https://github.com/thoughtbot/playbook/blob/master/book/one-weeks-iteration.md

Rather than maintain two versions and potentially cause confusion about
where to look for the process, I think we should stick to just the
playbook version.
make it more clear what do do instead of protected
correct language
Refine language.

When I read the original version I thought it was ambiguous: Should I avoid
defining protected methods, or calling them?
Update the spec example to use except syntax
Index foreign keys

https://tomafro.net/2009/08/using-indexes-in-rails-index-your-associations
Don't use instance variables in partials

* It is hard to reason about the source of instance variables in
  partials because the source is too far away.
* Like globals, they're not given to you by your caller, and
  maybe not even your caller's caller.
* That makes it hard to figure out what you're changing when you change the
  instance variable.
* It couples the partial to the place it's first used.
* If you use locals, then you can follow the chain all the way back.
Add guideline: avoid multiple assignments per line

* Reading requires scanning back and forth across the line
* Adding more assignments causes modifications instead of additions
* Hides complexity
* Easy to miss when scanning a method
Add point to avoid using semicolons in ruby
Combine testing into one heading
Fix typo in ERB style item
Replace URLs that redirect with their targets

* Removes short URLs
* Makes it easier to see what you're being linked to
* Improves readability by removing long, inline URLs
Add HTML, ERB rules regarding quotes and attributes

Prefer double quotes based on popular opinion and jQuery standard.
Review SCSS files for image-url use

As a Rails developer, I'm always looking for ways to be useful to
designers during code review of their HTML/SCSS changes. This is a good
one to look for. Hat tip @jessieay.
Add style guidelines on squashing and merging

* Merge commits add noise the Git log.
* A tree-based history is harder to follow than a linear log.
* Encourages small, cohesive commits.
Separate unit/acceptance test guides

* Move Acceptance Tests into their own section
* Move Factories into their own section
* Move Unit Tests into their own section
Style for dev and all environment seed data

* db/seeds.rb for data needed in all envrionments
* dev:prime rake task for dev environment seed data
Add functions to Sass style guide
Fix broken 'stubs and spies' link
Use "PascalCase" instead of ambiguous "CamelCase"

CamelCase usually refers to both camelCase as well as CamelCase.
PascalCase is a less ambiguous term.