Update documentation for Project overview (ReadMe)

[Sophietn]

Overview of what the project is aiming to achieve and why
Explanation of how the project fits within GSF theory of change

README.md must also contain this banner at the top:

[!important] Incubation Project This project is an incubation project being run inside the Green Software Foundation; as such, we *DON’T recommend using it in any critical use case. Incubation projects are experimental, offer no support guarantee, have minimal governance and process, and may be retired at any moment. This project may one day graduate, in which case this disclaimer will be removed.

[Sophietn]

From 28/11 agenda: Update to docs: Release schedule - "at most once per month, as frequent as possible" to update in README.md and CONTRIBUTING.md

If we need a security release - we prioritise that.
Documentation release can happen anytime.

[Sophietn]

Hey @vaughanknight you mentioned you've got a first draft for review on the call. Could you raise it please?