Reorganize the documentation

Will close issue #257 and #273 
What about explaining (quickly) that there are 2 fields:

1) Create a field (e.g. `imageName`) which will be stored to the database as a string. This will hold the filename of the uploaded file.

2) Create another field (e.g. `imageFile`). This will store the ``UploadedFile`` object after the form is submitted. This should *not* be persisted to the database, but you *do* need some extra annotations.