Add DCO check

[duglin]

Add a travis-ci check to ensure all PR's commits are signed with a DCO.

[MHBauer]

Can you add a definition of what this means?

[duglin]

@MHBauer Done

[MHBauer]

I'm not sure what you changed, but to be explicit:

1. define DCO
2. explain or link to what it means to agree to it
3. explain how to do it (git commit -s)
4. explain that signing off means you are agreeing to the dco

I can do research, but we shouldn't make people do research, or blow off the significance of what signing off is intending to mean.