Add roadmap

[zacharlie]

I have created a basic roadmap document. I think some of the issues highlighted need to be fleshed out and/ or formalized a bit so that community platform users are able to follow along with the development process and plan accordingly for when breaking changes are introduced

[timlinux]

Thanks so much for this @zacharlie

As a general strategy, lets add new docs in the sphinx tree rather and then if we want to leave some breadcrumbs the md files in the top level dir can just have pointers over into the sphinc docs. I will do that for this PR

[zacharlie]

I would expect high level docs like roadmap and contributing guidelines to be in the root of the repo myself. But if we rely on the docs for this then I think we just need to add links to the relevant docs in the readme.

I think for developer-centric elements, especially those that affect deployment, are critical to have in the readme.