Added note about uni-directionality of data-bind style bindings

[jacobSingh]

I spent an hour banging my head against this. I figure it's probably useful to someone else since the docs are not explicit.

[sarxos]

Hi Jacob, since Derick abandoned this project I assume this pull request won't be merged, however I included such a note in the project continuation:

https://github.com/oaprnd/backbone.modelbinding/commit/4d53298b4533afad5fb983a1c563b3c879726d4d

Complete description is available here:

http://oaprnd.github.com/backbone.modelbinding/

And project continuation here:

https://github.com/oaprnd/backbone.modelbinding

Cheers!