Bring contributing guidelines (including DCO info) from GitHub Working Group fully into this GitHub Org

[scottrigby]

We still have a bit of file moving to finish from GitOps Working Group to OpenGitOps, including the Contributor Guidelines. For now we have essentially a pointer to parts of the GWG Readme.

For example, for those who don't know about DCO, it's possible but not easy to find this with a bit of clicking from the OGO Contributing Guidelines file in this org's default community health files repo (https://github.com/open-gitops/.github), which links to the GitOps WG Readme, which has a Licenses and Copyright section at the bottom, which says:

Apache 2.0 is required for all git repositories
Developer Certificate of Origin (DCO) commit signoff is required for all new contributions

Let's bring that into the main OGO file properly.

In addition to this, we can help clarify a bit better for people. They may ask, what is DCO? We should point them to git help commit:

-s, --signoff
    Add Signed-off-by line by the committer at the end of the commit log
    message. The meaning of a signoff depends on the project, but it typically
    certifies that committer has the rights to submit this work under the same
    license and agrees to a Developer Certificate of Origin (see
    http://developercertificate.org/ for more information).

This is what I have done in the DCO GitHub UI browser plugin Readme, and I think we can basically just copy that info here.