[CONTRACT-69] Per-crate README documentation

[mftee]

Overview

None of the five crates (identity, shipment, escrow, document, reputation) has a README.md. [CONTRACT-36], "Per-crate README documentation," was closed on 2026-07-28 without landing — confirmed by find contracts -iname 'README*' returning nothing, the fourth confirmed never-landed closed contract issue in this batch (after [CONTRACT-24], [CONTRACT-23], [CONTRACT-25]/[CONTRACT-30]/[CONTRACT-35]'s equivalents). A new contributor has to read raw lib.rs source to understand what each contract does, its entrypoints, and its relationship to the others.

Tasks

• Add a README.md to each of the five crates covering: purpose, public entrypoints with a one-line description each, the DataKey storage layout, and — once [CONTRACT-38]/[CONTRACT-39]/[CONTRACT-40] land — its cross-contract relationships.
• Add a top-level contracts/README.md (if one doesn't already effectively exist via the root README.md's brief mention) summarizing how the five crates fit together as a system.

Acceptance Criteria

• Every crate has a README a new contributor can read instead of the raw source to understand its purpose and API.

Notes for Contributors

Requires Rust + Soroban. Workspace: contracts/ (members: identity, shipment, escrow, document, reputation). Run cargo test --all, cargo fmt --all -- --check, and cargo clippy --all-targets --all-features -- -D warnings before opening a PR — CI enforces all three. Comment below to be assigned.