docs: add contributing guide

CONTRIBUTING.md

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+# How to contribute
+
+## Getting started
+
+- Fork the repository on GitHub
+- Checkout the fork and create a branch for you to work on
+- This project uses [projen](https://github.com/projen/projen) as a management tool
+- Run `npm ci` once, then `npm build`
+
+## Testing packages for other languages
+
+Like every Construct, *cdk-esbuild* is a [jsii](https://github.com/aws/jsii) project and a package is published for additional language.
+Sometimes it is required to test these generated packages in a real life environment.
+All paths in the instructions below, will assume you are testing with one of the examples.
+
+### NodeJs
+
+**Option 1:**
+
+*This is the preferred approach, as it is more consistent and closer to how npm would behave for a real user.*
+
+- `pj build`
+- The NodeJS package can be found in `dist/js`
+- In your Python app, run `npm install ../../dist/js/[email protected]` (path to the file in dist)
+- `npx cdk synth` will use the locally build version
+
+**Option 2:**
+
+*This approach allows for faster iterations. However it is not a true representation of what would happen for a real user. There are known issues with CDK version mismatches.*
+
+- In `package.json` add/change `@mrgrain/cdk-esbuild: '../..'` (path to your)
+- `npx cdk synth` will use the compiled files in `lib`
+
+### Python
+
+- `pj build`
+- The Python package can be found in `dist/python`
+- In your Python app, run `pip install -I ../../dist/python/mrgrain.cdk_esbuild-0.0.0-py3-none-any.whl` (path to the .whl file)
+- `npx cdk synth` will use the locally build version