add ROADMAP.md

Signed-off-by: Victor Vieux <vieux@docker.com>
Merge pull request #162 from docker/roadmap

add ROADMAP.md
First step toward modular discoevery services

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

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