Add documentation guidance/instruction to to site subfolder.

[ADubhlaoich]

User Story
As a person interested in contributing to documentation

I want to know how to write and test documentation changes

So that I can create improvements to the documentation

Acceptance Criteria
The site subfolder should have a README explaining how to use Hugo

The root README should direct people to the site README for doc contributions

[ADubhlaoich]

I'm taking this task up. Most of the heavy lifting for this already exists in the repository for NGINX Ingress Controller: I will copy that README, make appropriate updates for a PR here, then add any improvements to their repository.

[mpstefan]

Completed by #1431