CONTIBUTING.md: Add guidelines for contributions

This follows the CONTRIBUTING.md found in other ROS projects with
a change to the MIT License to match meta-ros and the Yocto Project.

Signed-off-by: Rob Woolley <[email protected]>

Author: robwoolley

CONTRIBUTING.md

@@ -0,0 +1,8 @@
+Any contribution that you make to this repository will be under the 
+[MIT License](https://opensource.org/license/mit/), unless explicitly stated
+otherwise.
+
+Contributors must sign-off each commit by adding a `Signed-off-by: ...`
+line to commit messages to certify that they have the right to submit
+the code they are contributing to the project according to the
+[Developer Certificate of Origin (DCO)](https://developercertificate.org/).