docs: clarify that no action regarding the DCO is required
docs: clarify that no action regarding the DCO is required

For incoming contributions we assume their authors accept the DCO. No
further action from new developers is required. An added sentence in
the guidelines clarifies this.

Fixes #3193

weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope