Add contributing dir with Config file checklist

[banks]

This checklist was reviewed internally in a document form a few months ago but I wanted to make it public.

I've followed Nomad's conventions of adding a contributing dir. The only quirk here is we already have a .github/CONTRIBUTING.md which has some overlap. GitHub links to that in new PRs so it's useful to surface there and will only pick it up from .github in the root or in /docs/ dir. I don't want to add /docs and choose not to move it for now as we don't want other checklists in the root so it still doesn't end up in the same place but open to other options here.

I tried to make this a relatively minimal change and we can certainly iterate on the README and other things.

The contributing/README.md is based heavily on the Nomad one: https://github.com/hashicorp/nomad/tree/master/contributing for now. I don't know that they need to be in perfect harmony so we can iterate but it seemed like a decent starting point.

Finally I moved INTERNALS.md to avoid having a sprawl of things in different places I left a stub behind so links don't break in case anyone links to it in mailing list issues etc.

[crhino]

Overall, I like this idea a lot. I think it will help distribute knowledge and enable better contributions.

I especially thought the explanations of why/when you would need to go through each of the four cases presented was useful to know whether I needed to do it or not.

I had to do the first 2 steps recently with hclog, so I mimicked running through this checklist with that change. I found that using a gist was not the greatest UX, since it only showed me the raw markdown. I ended up copying it again in a comment below (https://gist.github.com/crhino/6cefaa4a1f06aebbcc3ef274f3b06b58). Not sure whether to keep that suggestion in or not.

I also liked having the exact file locations pointed out, and I added a few comments updating some of the bullet points with full paths.

[banks]

Great feedback @crhino thanks!

I found that using a gist was not the greatest UX, since it only showed me the raw markdown.

If you name the "file" something that ends in *.md then gists will format it as Markdown and include clickable check boxes: https://gist.github.com/banks/35e4af7feb4c8f0db7a8a645de29bde9 (you probably need to fork that to make the checkboxes clickable as it edits the underlying markdown).

Do you think it would help to be more explicit about that in the README?

[crhino]

Ah, good to know that gists can do that! I don't think it is necessary to go into that much detail in our README, the essential point is to include the checklist with the PR in my opinion, the workflow used to get there doesn't have to be too detailed.

[crhino]

LGTM