Summarise this docs site in readme.md

[ndjones]

Summarise this site, who its for, etc in the readme.md

[ndjones]

This looks great, thanks Kahu.

@junhuhnesi I wonder if we want to take this a little further, to link into our broader view of the platform itself, which the mkdocs site is capturing for the product. something that encourages a broader sense of engagement, along with pointers to how to raise service requests, etc e.g. https://github.com/opengovernment/opengovernment/blob/master/CONTRIBUTING.md