Clean up Bundler guidelines

* Remove old Bundler recommendation in Rails section, which has been
  superceded by the Bundler section.
* Alphabetize Bundler guidelines.
* Wrap at 80 characters where possible.
* Add binstubs guideline.
Add database index guidelines

Extracted from old thoughtbot development handbook.
Improve readability of "Index columns" guidelines
Clarify foreign key index guideline

More often than not, this is a good idea but not a 90% rule.

For example, if a users table has a subscription_id column, you might
need to look up the related subscriptions row (`SELECT * FROM
subscriptions WHERE id = ?`) but never do the opposite (`SELECT
* FROM users WHERE subscription_id = ?`).

Every unused index hurts performance and makes `EXPLAIN` statements
harder to understand.