Add note to README about using absolute paths

This issue bit me for a while until I started searching through old issues and found these issues:

- https://github.com/jeffknupp/sandman/issues/34
- https://github.com/jeffknupp/sandman/issues/46

This commit seemed to make this issue more clear but I guess this was lost when switching to the click library for command-line parsing.
  - https://github.com/jeffknupp/sandman/commit/34406d3797dd567647b78e0cf6a030a4299d68fa