Add upgradeability pattern doc for contracts

[joelpeace48-cell]

Document how we could make the rewards or campaign contract upgradeable (e.g. deployer pattern, migration steps) and add a short "Future: upgradeability" section in docs.

[D2758695161]

Crab Bounty Claim

Submitted PR #135 for this issue. Payment address: 0xaae0101ac77a2e4e0ea826eb4d309374f029b0a6