Add extra info about DCO check to README
Add extra info about DCO check to README

We recently added the DCO github app and it will get run on every PR. This change
adds a small blurb to our README to give more information on DCO for contributors.

Signed-off-by: Daniel Canter <8d0fd95ae71f7e4870b010b10f83a6201b034a65@microsoft.com>

Microsoft/hcsshim