Documentation does not properly explain requirements

[basicallydan]

This is a great plugin, but in order to make it work, I believe there are a few CSS requirements for the markup which are not documented, which slowed me down quite a bit given that the markup I was working with was considerably more complex than the

• The container to which the plugin is attached must be an absolutely-positioned element with 100% height
• The parent of the container must, therefore, be relative or absolutely positioned.
• The parent must also be 100% height

If these requirements are not satisfied, the calculation of the movement deltas are a little screwed up.

I will get around to making a pull request for this soon :)