OME DCO

This PR adds documentation and a utility hook to comply with OME upcoming DCO policy:
- adds a section to `CONTRIBUTING.md` about the DCO policy
- adds a section to `Checking out the source code` section of the contributing documentation
- adds a pre-commit hook to the `downloads` folder of the contributing documentation

NB: Until this PR is merged, all absolute URLs from this PR will NOT work and you should use the following URL to try the hook setup https://raw.githubusercontent.com/openmicroscopy/ome-documentation/develop/contributing/downloads/prepare-commit-msg.hook