Add CONTRIBUTING.md

Author: arnautov-anton

.github/workflows/CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing to this project! We appreciate your help in making this project better.
+
+## Testing Changes
+
+Before submitting your pull request, please ensure that your changes do not break the build. To test your changes:
+
+1. Run the following command in the root folder of the project to build the demo applications:
+
+   ```bash
+   npm run build:js:public
+   ```
+
+2. After the build process is successful, try to host the project locally using a tool like [`http-server`](https://npmjs.com/http-server):
+
+   ```bash
+    http-server docs/
+   ```
+
+3. Open your web browser and navigate to `http://localhost:8080` (or the port specified by `http-server`) to view the hosted project. Click through the generated applications and make sure they work.
+
+## Submitting Changes
+
+Once you have made and tested your changes:
+
+1. Commit your changes with a descriptive commit messages.
+2. Push your changes and create a PR summarizing the changes you've made.
+3. Wait for an approval and then **squash-merge** to the `main`, the CD will take care of the deployment process.
+4. Make sure the deployment job was successfull by visiting `https://getstream.io/chat/demos`, test your changes there as well. Unsuccessful jobs won't publish, debug accordingly.