Add rule avoid `any_instance` in RSpec.

* `any_instance` promotes depending on concretions.
* `any_instance` hides complexity.
* Pure stubs promotes isolation and dependency injection.
* Using dependency injection promotes inversion of control.
Add more indepth style for Sass
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 cheat-sheets
Create README.md
Update README.md
Delete capybara.md
Delete tmux.md
Update README.md
Update README.md

Add virtualenv
Create heroku.md

Work-in-progress
Update README.md
Update heroku.md
Update heroku.md