Improve `body`/`body_path` documentation

[hmontone]

README.md:

body: Text describing the contents of the release. Optional, and not needed if using body_path. body_path: A file with contents describing the release. Optional, and not needed if using body.

Of course body is not needed if using body_path and body_path is not needed if using body, since they are optional according to the documentation. What would be useful, however, is to know what happens when both or neither of them is defined.

[hmontone]

How about something like:

`body`: Text describing the contents of the release. If `body_path` is given, its contents will take precedence. If neither `body` or `body_path` is given, the release will be created with an empty body.
`body_path`: A file with contents describing the release. See also `body`.

[tavrez]

Please also add how should we address the file for body_path, absolute or relative, and relative to where