docs(contrib): add CONTRIBUTING.md with baseline burndown policy (#139)

* docs(contrib): add CONTRIBUTING.md with baseline burndown policy

* fix: clarify wire-shape gate path-filter (not every PR)

Author: saurabhjain1592

CONTRIBUTING.md

@@ -0,0 +1,34 @@
+# Contributing to AxonFlow Java SDK
+
+Thank you for your interest in contributing! Please open an issue or pull request via the [GitHub repository](https://github.com/getaxonflow/axonflow-sdk-java).
+
+## Development setup
+
+```bash
+./mvnw verify
+```
+
+Tests run under JUnit 5. The wire-shape contract gate (under `scripts/wire_shape/`) runs in CI when a PR touches Java sources, the wire-shape baseline, or the gate scripts themselves — see `.github/workflows/wire-shape-contract.yml` for the exact path filter.
+
+## Pull request guidelines
+
+1. Keep PRs focused — one feature or fix per PR.
+2. Update `CHANGELOG.md` under `[Unreleased]` for user-visible changes.
+3. Ensure `./mvnw verify` is green and the wire-shape contract gate passes.
+4. Prefer Jackson `@JsonProperty` on a `@JsonCreator` constructor for any new wire-bound POJO field — that propagates serialization both ways without hand-rolled transformer code.
+
+## Baseline burndown policy
+
+The wire-shape contract gate uses a baseline file (`tests/fixtures/wire-shape-baseline.json`) to grandfather pre-existing drift findings — the gate fails on any *new* drift but tolerates the listed entries. The baseline exists to land the gate without a giant cleanup PR; it is not intended to be permanent.
+
+When your PR touches a type listed in the baseline, do one of:
+
+- **Burn it down.** Realign the POJO with the OpenAPI spec in this PR, regenerate the baseline via `scripts/wire_shape/refresh.py`, and note "burndown: `<entry>`" in the PR description.
+- **Justify it.** If the drift can't be resolved in this PR (different scope, blocked on a platform spec change, etc.), say so in the PR description in one line.
+
+CI does not block PRs that touch a baselined type without addressing it, but reviewers will ask the burndown-or-justify question.
+
+## Questions
+
+- Open a GitHub issue
+- Email: hello@getaxonflow.com