Backwards compatibility document

[cmwylie19]

Is your feature request related to a problem? Please describe.

Define and document the backwards-compatibility rules that apply to Pepr for minor and releases.

Describe the solution you'd like

Similar to https://github.com/helm/community/blob/main/hips/hip-0004.md

• Given you are on the Pepr repo
• When searching for how we handle backwards compatibility
• Then the answers are found in a document

Describe alternatives you've considered

(optional) A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.