Add specific CONTRIBUTING.md file

Per https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/183#note_13922336 , the plan is to have each project have its own CONTRIBUTING.md, but then from there link to the CLA's in the gitlab-ce repo.

CONTRIBUTING.md

+## Contributing to GitLab Development Kit
+
+The source of GitLab Development Kit is [hosted on
+GitLab.com](https://gitlab.com/gitlab-org/gitlab-development-kit/) and
+contributions are welcome!
+
+## Contributor license agreement
+
+By submitting code as an individual you agree to the
+[individual contributor license agreement](https://gitlab.com/gitlab-org/gitlab-ce/doc/legal/individual_contributor_license_agreement.md).
+By submitting code as an entity you agree to the
+[corporate contributor license agreement](https://gitlab.com/gitlab-org/gitlab-ce/doc/legal/corporate_contributor_license_agreement.md).