Add CONTRIBUTING.md

CONTRIBUTING.md

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+## Modifying/Adding code
+
+Most of the SDK is generated code. Modifications to code will be persisted between generations, but may
+result in merge conflicts between manual patches and changes from the generator. The generator will never
+modify the contents of the `arcade-java/lib/` and `arcade-java-examples/` directories.
+
+## Adding and running examples
+
+TODO
+
+## Building the repository from source
+
+If you’d like to use the repository from source, you can build and install it from git:
+
+```sh
+git clone git@github.com/ArcadeAI/arcade-java.git
+SKIP_MOCK_TESTS=true ./gradlew build publishToMavenLocal # This will skip the integration tests, see below on info on how to run them.
+```
+## Running tests
+
+Most tests require you to [run a mock server](https://github.com/stoplightio/prism) against the OpenAPI spec to run the tests.
+
+```sh
+# you will need npm installed
+./scripts/mock --daemon
+```
+
+The mock serer will run in the background allowing you to run tests from the command line `./gradlew test` or from your IDE.
+
+To stop the mock server run:
+```shell
+pkill -f prism
+```
+
+## Linting and formatting
+
+This repository uses [Spotless](https://github.com/diffplug/spotless/tree/main/plugin-gradle) to lint and format the code in the repository.
+
+To lint:
+
+```sh
+$ ./scripts/lint
+```
+
+To format and fix all issues automatically:
+
+```sh
+$ ./scripts/format
+```