add matching to constraints

Signed-off-by: Victor Vieux <vieux@docker.com>
add tests and move from label to constraint

Signed-off-by: Victor Vieux <vieux@docker.com>
add README.md

Signed-off-by: Victor Vieux <vieux@docker.com>
update docs

Signed-off-by: Victor Vieux <vieux@docker.com>
simplify code

Signed-off-by: Victor Vieux <vieux@docker.com>
update main README.md

Signed-off-by: Victor Vieux <vieux@docker.com>
update README.md

Signed-off-by: Victor Vieux <vieux@docker.com>
Merge pull request #186 from vieux/improve_constraints

Improve constraints to add matching