First draft of RSpec guidelines

***Don't merge***

Just looking for feedback.
I don't think only half-splitting things like this across multiples makes it very easy to read. With things like this, one of the two I think is best

```ruby
allow(AssociatedClass).to receive(short_variables).with(rspec_3_syntax).and_return multiline_variables_use_do_end_syntax
``` 

```ruby
allow(AssociatedClass).
  to receive(short_variables).
  with(rspec_3_syntax).
  and_return multiline_variables_use_do_end_syntax
```

So, we either put the whole thing on a single line or split the whole thing multiple lines.

Only wrapping the last components (the `and_return` in this case) seems like doing this when you reach the end of a sentence:

> This is me writing a sentence, but I run out of space before the end so I just
>                                                                                cram the last words in here