Add comment to all files stating the origin

[rgaiacs]

Is getting more common for contributors to do changes in files originated in this repository but send the pull request to another place, for example, https://github.com/swcarpentry/workshop-template/pull/458. Contributors are not to be blame!!! They are doing it on their best intentions. We need to add at the top of every file stating that the changes need to be send to this repository.