docs: add '--patch-dir' to command-line help
docs: add '--patch-dir' to command-line help

It is a viable solution for some issues but was not visible in help
so people were not aware of it

Mentioned here: https://github.com/ds300/patch-package/issues/203#issuecomment-618357240
ds300/patch-package,ds300/patch-package,ds300/patch-package