Update contribution page

[NomadicCore]

A quick write up to provide something for our contribution page from which we can develop and iterate.

[malfisya]

Just a little suggestions :)

[CookieSource]

I think it would be better if the reference

1. Make a localy copy of the repo
2. Create an update branch and build the site locally
3. Make changes as appropriate and ensure those changes do not break the site
4. Once you are ready, submit a PR to the repo.

Would instead just link to the readme of the repo as having instructions there make more sense so we can give instructions per project. Though this could also be on the wiki.

[ermo]

Just a few nits, otherwise looks good to me.