Add proposal process

[nikimanoledaki]

What type of PR is this?

kind/documentation

What this PR does / why we need it:

• Creates a proposal process through this repo
• Changes to proposals / design docs can be tracked through git
• Adds template with instructions (inspired by KEP/ADRs)
• Helps to deprecate the design doc (which doesn't scale well)

Which issue(s) this PR fixes:

https://github.com/cncf-tags/green-reviews-tooling/issues/47

Special notes for your reviewer (optional):

Please let me know what you think about creating a proposal process through this repository. It's similar to the KEP process in Kubernetes or ADRs which is widely used in OSS. Going through git would allow a better tracking of the history of proposals and how they change. It would also help to have discussions on individual points via PRs.

[dipankardas011]

Hi

[leonardpahlke]

Having some kind of design document with the usual questions is a good change. The KEP template is quite extensive. We will see which sections will make sense once we use it a few times. Lets try this out and make adjustments if needed.