Donation Resource description update

@arionhardison @DavidHuie I suggest you click on the 'View' button to check the end result instead of looking at the diff.

I took up where Arion left off, this pull request is for my changes only.

* add footnotes
* use the term 'writable' instead of 'read-only'
* corrections in the writable and required columns, complete missing example values
* encourage clients not to rely on the defaults which are somewhat brittle since they are the result of various callbacks on the model. At this point it is not an option to make these fields required so a strong recommendation is my best shot.
* update the creation example to promote better usage of the endpoint