Document script working directory and environment variables

[jonhoo]

In certain pre/post scripts, it is useful to be able to refer back to files in the directory that houses that script itself (e.g., static assets, configuration files, etc.). It would be great if the working directory at the time when image-bootstrap is invoked was somehow made accessible to scripts (perhaps through an ORIGINAL_WD environment variable? An alternative would be to just chdir back to the original working directory before invoking the scripts; I don't know which is better.

[jonhoo]

Ah, it seems like the working directory is preserved. This should probably be documented somewhere!