docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Keep PRs focused and include the smallest checks that cover the change.
+
+## Setup
+
+Install Dart 3.10 or newer and Rust stable. Install Flutter stable when working
+on `bdk_demo` or mobile targets.
+
+From the repository root:
+
+```bash
+dart pub get
+```
+
+For the Flutter demo:
+
+```bash
+cd bdk_demo
+flutter pub get
+```
+
+## Bindings
+
+The generated Dart bindings live in `lib/bdk.dart`. Do not edit that file by hand.
+When changing native Rust inputs or the pinned `bdk-ffi` dependency, regenerate
+bindings and the native library:
+
+```bash
+bash ./scripts/generate_bindings.sh
+```
+
+## Checks
+
+These shortcuts require `just`; `just ci` runs the full local format, analysis,
+test, and demo check set.
+
+Before opening a PR, run the relevant subset:
+
+```bash
+just format
+just analyze
+just test
+```
+
+If you touch `bdk_demo`, also run:
+
+```bash
+just demo-analyze
+just demo-test
+```
+
+Integration tests are skipped unless enabled. See `README.md` for
+`BDK_DART_RUN_INTEGRATION`, Electrum, and Esplora environment variables.
+
+## Pull requests
+
+Use clear branch names and commit messages. In the PR description, include a
+short summary, any relevant issue link, and the checks you ran.
+
+CI runs format, analysis, tests, binding generation, and platform smoke builds.