add DCO to contribution guide

After @beorn7 said that the DCO is referenced in the contribution guides of most Prometheus projects I noticed it was _not_ in the pushgateway so here's a tiny update. Signed-off-by: David Worth <[email protected]>
prometheus · Oct 28, 2019 · 22b9bc3 · 22b9bc3
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diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -16,3 +16,5 @@ Prometheus uses GitHub to manage reviews of pull requests.
   and the _Formatting and style_ section of Peter Bourgon's [Go: Best
   Practices for Production
   Environments](http://peter.bourgon.org/go-in-production/#formatting-and-style).
+
+* Be sure to sign off on the [DCO](https://github.com/probot/dco#how-it-works)