Document the availability of workflow.basedir and workflow.current_basedir

[pvandyken]

As a followup to #102, Snakemake offers a few properties to access the app root directory. This is useful for getting resources etc stored in the app dir, as Snakebids apps often have their data in a different working directory than the app dir, so relative file paths don't work. The presence of these properties is not well documented in the Snakemake docs, but given their special relevance to Snakebids projects, it would be appropriate to give them special mention in our own docs.