Write down design principles and non-goals

[darthfork]

Mature rulesets usually explain their intended scope so maintenance stays sane.

Tasks:

• Add a short design document.
• State what the ruleset will support in v0.x and what it intentionally will not.
• Decide whether the project wants a small-core philosophy or a more batteries-included direction.

Acceptance criteria:

• Contributors can evaluate new features against explicit project goals.

Parent issue: #6