Update simple.html.erb.md

If captions is good practice then we should promote it's use. It is also too confusing to have different points be covered across different examples. You could add to the "tips and FAQ" page something about "other ways for providing captions", though I'm not sure we should even list them. If more discussion is needed around captions then consider making a separate page as it previously was.
Merge pull request #31 from nitedog/patch-2

Making clear that captions are _not_ required but best practices.