As discussed, Python modules which aren't intended to be invoked
as scripts should not include a shebang line.
Update CONTRIBUTING.md and the checker script.
This script now includes a list SKIP_FILES for files that
should not be checked for boilerplate template.
The tests will now fail if a Python module has a shebang line.
Scripts which should have a shebang line and exists in the directory
`hack` can be ignored by adding them to the SKIP_FILES list.
