Add docstring for define-resource

[jgarte]

This is an issue to add basic documentation/doc string to define-resource besides the code:

https://github.com/charJe/portal/blob/master/portal.lisp#L38

Let's capture our thoughts about it here and then if you'd like I can open a PR to add it in from the information I gather.

[jgarte]

@charJe Can we label all these issues related to documentation with the documentation label that GitHub provides?