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
hack/PACKAGERS.md: libdevmapper
Factor cache-probing logic out of buildfile.commit() and CmdRun().
Record added-tree sha256 in buildfile.CmdAdd, probe and use cache.
Add testcases for ADD caching, closes #880.
Add self to AUTHORS.
Add CHANGELOG.md entry for ADD caching.
Add -S option to tar for efficient sparse file handling

Fixes issue #3282
make a more complete example of docker run, showing the use of most of the options (Closes #1500)
Return error if Dockerfile is empty
Return meaningful error for meaningless Dockerfiles
Add arch/os info to user agent (Registry)
Add dynamic os/arch detection to Images
Allow mtu to be configured at daemon start
Only parse upto the mountpoint in mountinfo
Merge pull request #3294 from discordianfish/3293-better-error-dockerfile-empty

Return error if Dockerfile is empty
Merge pull request #3291 from dineshs-altiscale/3282-sparse-files

Add -S option to tar for efficient sparse file handling
Merge pull request #3300 from crosbymichael/fix-mountinfo-parsing

Only parse upto the mountpoint in mountinfo
Merge pull request #3289 from crosbymichael/add-mtu-option

Allow mtu to be configured at daemon start