Introduce contribution/backports policy

[sam-heilbron]

The projects within kgateway-dev are mature and now support multiple patch streams.

For example, in kgateway-dev/kgateway there is:

• https://github.com/kgateway-dev/kgateway/tree/v2.0.x
• and a 2.1.x branch will soon be cut from https://github.com/kgateway-dev/kgateway/tree/main

Without a policy clearly defining who steps need to be taken to backport changes, there is risk to instability in patch streams