wiki: add Contributing page

Contributing.md

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+# Contributing
+
+---
+
+## Dev setup
+
+```bash
+git clone https://git.gabrielkaszewski.dev/GKaszewski/k-ap.git
+cd k-ap
+cargo build
+```
+
+No special tooling required beyond a recent stable Rust (edition 2024).
+
+---
+
+## Make targets
+
+```bash
+make check    # rustfmt --check + clippy -D warnings + tests
+make fmt      # apply rustfmt
+make fix      # fmt + clippy --fix (auto-fixable lints)
+```
+
+Always run `make check` before pushing. CI runs the same checks.
+
+---
+
+## Before opening a PR
+
+- `make check` passes with zero warnings
+- New behavior has test coverage — tests live in `src/tests/`
+- No unrelated refactoring mixed into the PR
+- Commit messages follow imperative style: `feat: add X`, `fix: Y`, `refactor: Z`
+
+---
+
+## Project structure
+
+```
+src/
+├── activities/    # One file per AP activity type (Follow, Create, Announce, ...)
+├── repository/    # Trait definitions (ActivityRepository, FollowRepository, ...)
+├── service/       # Service methods (broadcast, follow management, delivery, backfill)
+├── tests/         # Unit and integration tests
+├── content.rs     # ApContentReader, ApObjectHandler traits
+├── user.rs        # ApUserRepository, ApUser, ApVisibility, LookedUpActor
+├── data.rs        # FederationData, EventPublisher, FederationEvent
+└── lib.rs         # Public API surface — all re-exports
+```