Specify when to use the different string types

We *should* be using single quotes for everything, unless we need to use
either a double-quote or percent notation. This defines rules to
follow which guide the decision for which string type to use.
I know this has been given a thumbs up already but was curious what an example would look like for this case: 
*"Use double quotes when there is interpolation and either double quotes OR single quotes."*

Also, should there be an example of when we want to escape special characters?
```ruby
#bad
value5 = 'hello\nworld'

#good
value5 = "hello\nworld"
```