add section on early returns

@ColinDKelley I got this feedback on my PRs before. how do we feel about adding it to the style guide?
I think this pattern smells a bit too.  First off, you're using trailing `if` for a presumably rare case, which elsewhere in this guide we say not to do; the rule there is that the statement should execute almost always so that the trailing `if`/`unless` can be viewed as a footnote.  But secondly, returning from a method without doing any work is often a sign of a bad contract where the caller should really have not called the method.  If you clear the contract hurdle, I'd still prefer an `if` to an early `return` almost always.  I sometimes hear "but I don't want to indent all 40 lines of my method".  That objection rarely comes up when methods are <= 5 lines. :)
Assuming the "don't care" case is rare in your example, it would ironically be a good candidate for the trailing `if`!
```
def my_method
  work_i_do_in_most_cases(x) unless i_dont_care_about_this_case
end
```
That's much clearer don't you think?  You'll see code like that all over Rails.