Add Developer Certificate of Origin Text

We purposely chose to license Docker under the Apache 2.0 license. This is a well-known
license, which is popular for its permissive and flexible properties, while still
encouraging a collaborative community. It also makes certain representations with regard
to contributions, and the rights given to contributors, the project, people who use Docker,
people who modify Docker, etc.

This approach to contributions is inspired by the popular Linux Developer
"Certificate of Origin". This approach makes it simple for new contributors to get started,
and avoids bureaucracy in tracking contributions and contributors.

To indicate accordance, each individual contribution to the Project is signed off by the
developer, using his or her real name, email address, and github handle in the format below:

	Docker-DCO-1.0-Signed-off-by: Joe Smith <joe.smith@email.com>
Merged with master
Fix dotCloud to Docker, Inc
Remove the @github handle as it generates emails.
Merge pull request #2091 from keeb/add-dco

Add Developer Certificate of Origin Text