Update CONTRIBUTING.md for DCO, test, and lint

CONTRIBUTING.md

@@ -6,7 +6,37 @@ Prometheus uses GitHub to manage reviews of pull requests.
   addressing (with `@...`) the maintainer of this repository (see
   [MAINTAINERS.md](MAINTAINERS.md)) in the description of the pull request.
 
-* If you plan to do something more involved, first discuss your ideas
-  on our [mailing list](https://groups.google.com/forum/?fromgroups#!forum/prometheus-developers).
-  This will avoid unnecessary work and surely give you and us a good deal
-  of inspiration.
+* If you plan to do something more involved, first discuss your ideas on
+  [our mailing list]. This will avoid unnecessary work and surely give you and
+  us a good deal of inspiration.
+
+* Before your contributions can be landed, they must be signed off under the
+  [Developer Certificate of Origin] which asserts you own and have the right to
+  submit the change under the open source licence used by the project.
+
+## Testing
+
+Submitted changes should pass the current tests, and be covered by new test
+cases when adding functionality.
+
+* Run the tests locally using [tox] which executes the full suite on all
+  supported Python versions installed.
+
+* Each pull request is gated using [Travis CI] with the results linked on the
+  github page. This must pass before the change can land, note pushing a new
+  change will trigger a retest.
+
+## Style
+
+* Code style should follow [PEP 8] generally, and can be checked by running:
+  ``tox -e flake8``.
+
+* Import statements can be automatically formatted using [isort].
+
+
+[our mailing list]: https://groups.google.com/forum/?fromgroups#!forum/prometheus-developers
+[Developer Certificate of Origin]: https://github.com/prometheus/prometheus/wiki/DCO-signing
+[isort]: https://pypi.org/project/isort/
+[PEP 8]: https://www.python.org/dev/peps/pep-0008/
+[tox]: https://tox.readthedocs.io/en/latest/
+[Travis CI]: https://docs.travis-ci.com/