docs: add contributing.md

contributing.md

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+# Contributing to FingerprintJS Pro SPA integration
+
+## Working with code
+
+We prefer using [yarn](https://yarnpkg.com/) for installing dependencies and running scripts.
+
+The main branch is locked for the push action. For proposing changes, use the standard [pull request approach](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request). It's recommended to discuss fixes or new functionality in the Issues, first.
+
+### Development playground
+
+It's a demo page in `/examples/spa` folder. More info about configuring and starting demo you can find in [readme](examples/spa/README.md).
+
+If you want to test integration with [fingerprintjs-pro-spa](https://github.com/fingerprintjs/fingerprintjs-pro-spa), just [link package](https://yarnpkg.com/cli/link) with the `yarn link`.
+
+❗ Build projects before testing integration. First build `fingerprintjs-pro-spa`, after `fingerprintjs-pro-react` and then start spa example app.
+
+### How to build
+Just run:
+```shell
+yarn build
+```
+
+### Code style
+
+The code style is controlled by [ESLint](https://eslint.org/) and [Prettier](https://prettier.io/). Run to check that the code style is ok:
+```shell
+yarn lint
+```
+
+You aren't required to run the check manually, the CI will do it. Run to fix code style mistakes (not all mistakes can be fixed automatically):
+```shell
+yarn lint:fix
+```
+
+### How to test
+Tests located in `__tests__` folder and run by [jest](https://jestjs.io/) in [jsdom](https://github.com/jsdom/jsdom) environment.
+
+To run tests you can use IDE instruments or just run:
+```shell
+yarn test
+```
+
+To check the distributive TypeScript declarations, build the project and run:
+```shell
+yarn test:dts
+```
+
+### How to publish
+- Create a new branch
+- Run `yarn release:(major|minor|patch)` depending on the version you need
+- Make a pull request
+- After merging the pull request into the main branch and after successful tests, GitHub Action will publish a new version to the npm