Add contributing guide and link to docs from README

CONTRIBUTING.md

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+# Contributing to JSON P3
+
+Hi. Your contributions and questions are always welcome. Feel free to ask questions, report bugs or request features on the [issue tracker](https://github.com/jg-rp/json-p3/issues) or on [Github Discussions](https://github.com/jg-rp/json-p3/discussions).
+
+**Table of contents**
+
+- [Development](#development)
+- [Documentation](#documentation)
+
+## Development
+
+The [JSONPath Compliance Test Suite](https://github.com/jsonpath-standard/jsonpath-compliance-test-suite) is included as a git [submodule](https://git-scm.com/book/en/v2/Git-Tools-Submodules). Clone the JSON P3 git repository and initialize the CTS submodule.
+
+```shell
+$ git clone [email protected]:jg-rp/json-p3.git
+$ cd json-p3
+$ git submodule update --init
+```
+
+We use [npm](https://docs.npmjs.com/cli/v10/commands/npm) to mange packages and run scripts during development. Install development dependencies with:
+
+```shell
+$ npm install --production=false
+```
+
+And run tests with the _test_ script.
+
+```shell
+$ npm test
+```
+
+Check for linting errors with the _lint_ script.
+
+```shell
+$ npm run lint
+```
+
+And check for typing errors with the _type-check_ script.
+
+```shell
+$ npm run type-check
+```
+
+Generate an HTML test coverage report with the _coverage_ script.
+
+```shell
+$ npm run coverage
+```
+
+Then open `coverage/index.html` in your browser.
+
+Build distribution bundles and generate `.ts` files with the _build_ script. This will write bundles to the `dist/` folder.
+
+```shell
+$ npm run build
+```
+
+## Documentation
+
+[Documentation](https://jg-rp.github.io/json-p3/) is built with [Docusaurus](https://docusaurus.io/). Find its source in the `docs/` folder of the main branch. Start the Docusaurus development server from within the `docs/` folder.
+
+```shell
+$ cd docs
+$ npm run start
+```