chore: add contribution guide

CONTRIBUTING.md

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+# Contributing
+
+## Reporting Bugs
+
+If you've found a bug with SVGO, [create an issue](https://github.com/svg/svgo/issues) on GitHub.
+
+Your issue should ideally contain:
+
+* A concise description of the bug.
+* How you were using SVGO, the version of the tool, and any configuration or command-line options.
+* The SVG that was effected, or a [Minimal, Reproducible Example](https://stackoverflow.com/help/minimal-reproducible-example).
+
+If you haven't found a bug, but need help using SVGO in your project, please consider asking on [Stack Overflow](https://stackoverflow.com/questions/tagged/svgo) with the `[svgo]` tag, you may get help faster there. You can still create an issue if the confusion stemmed from a lack of documentation.
+
+## Reporting Security Vulnerabilities
+
+See: [SECURITY.md](./SECURITY.md)
+
+## Development
+
+### Requirements
+
+* [Git](https://git-scm.com/)
+* [Node.js 14](https://nodejs.org/) or later
+
+### Getting Started
+
+Clone the repository with Git.
+
+```sh
+git clone https://github.com/svg/svgo.git
+```
+
+As this is a Node.js project and uses Yarn for package management, install the dependencies.
+
+```sh
+yarn install
+```
+
+Finally, make sure all quality assurance checks pass before making changes. This will lint, build, and test the project.
+
+```sh
+yarn run qa
+```
+
+### Plugins
+
+SVGO uses a plugin architecture, so we ultimately perform many smaller tasks rather than a single monolithic task. This provides users a lot of flexibility for which optimizations they use, and in what order to run them.
+
+See [`plugins/**`](./plugins/) for the list of existing plugins. This is where you can create new ones.
+
+You should read our [Plugin Architecture](https://svgo.dev/docs/plugins-api/) documentation for the gist of how to create a new plugin. If you've created custom plugins before, it's mostly the same process.
+
+#### Plugin Parameters
+
+SVGO plugins can optionally have parameters. These can be consumed by the plugin to tailor the behavior.
+
+As types are managed through TypeScript definition files and JSDocs, you must define the parameter types in [`plugins/plugin-types.ts`](./plugins/plugins-types.ts) for built-in plugins. Then you'll have code completion and type checking as you'd expect while editing the plugin if your code editor supports that.
+
+## Documentation
+
+Our documentation is maintained in [MDX](https://mdxjs.com/), which is Markdown with React components. The files are then pulled by [svg/svgo.dev](https://github.com/svg/svgo.dev) to build and deploy the SVGO website.
+
+To preview local changes, follow the steps to run the website locally in [svg/svgo.dev](https://github.com/svg/svgo.dev).
+
+New plugins, plugin parameters, and notable any features should be paired with documentation in the [`docs/`](./docs/) directory and pushed in the same pull request.
+
+## Funding
+
+Sponsoring the project helps keep it sustainable for current maintainers.
+
+See: [SVGO on Open Collective](https://opencollective.com/svgo)