Documented how layouts may cause an exception if you change your view's model type

Hopefully this will make it less likely that developers run into the issue discussed in this topic: http://our.umbraco.org/forum/templating/templates-and-document-types/38311-Using-RenderMvcController
Merge pull request #87 from michielvoo/patch-1

Documented how layouts may cause an exception if you change your view's ...