docs: add CONTRIBUTING.md (#1043)

* docs: add CONTRIBUTING.md

* docs(CONTRIBUTING.md): add period

* docs(CONTRIBUTING.md): unify the tone of the command part

CONTRIBUTING.md

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+# Contributing
+
+## General Guideline
+
+### Reporting Issues
+
+If you have found what you think is a bug, please [start a discussion](https://github.com/pmndrs/valtio/discussions/new?category=bug-report).
+
+For any usage questions, please [start a discussion](https://github.com/pmndrs/valtio/discussions/new?category=q-a).
+
+### Suggesting New Features
+
+If you are here to suggest a feature, first [start a discussion](https://github.com/pmndrs/valtio/discussions/new?category=ideas) if it does not already exist. From there, we will discuss use-cases for the feature and then finally discuss how it could be implemented.
+
+### Committing
+
+We are applying [conventional commit spec](https://www.conventionalcommits.org/en/v1.0.0/) here. In short, that means a commit has to be one of the following types:
+
+Your commit type must be one of the following:
+
+- **feat**: A new feature.
+- **fix**: A bug fix.
+- **refactor**: A code change that neither fixes a bug nor adds a feature.
+- **chore**: Changes to the build process, configuration, dependencies, CI/CD pipelines, or other auxiliary tools and libraries.
+- **docs**: Documentation-only changes.
+- **test**: Adding missing or correcting existing tests.
+
+If you are unfamiliar with the usage of conventional commits,
+the short version is to simply specify the type as a first word,
+and follow it with a colon and a space, then start your message
+from a lowercase letter, like this:
+
+```
+feat: add a 'foo' type support
+```
+
+You can also specify the scope of the commit in the parentheses after a type:
+
+```
+fix(react): change the 'bar' parameter type
+```
+
+### Development
+
+If you would like to contribute by fixing an open issue or developing a new feature you can use this suggested workflow:
+
+#### General
+
+1. Fork this repository.
+2. Create a new feature branch based off the `main` branch.
+3. Follow the [Core](#Core) and/or the [Documentation](#Documentation) guide below and come back to this once done.
+4. Run `pnpm run fix:format` to format the code.
+5. Git stage your required changes and commit (review the commit guidelines below).
+6. Submit the PR for review.
+
+##### Core
+
+1. Run `pnpm install` to install dependencies.
+2. Create failing tests for your fix or new feature in the [`tests`](./tests/) folder.
+3. Implement your changes.
+4. Run `pnpm run build` to build the library. _(Pro-tip: `pnpm run build-watch` runs the build in watch mode)_
+5. Run the tests by running `pnpm run test` and ensure that they pass.
+6. You can use `pnpm link` to sym-link this package and test it locally on your own project. Alternatively, you may use CodeSandbox CI's canary releases to test the changes in your own project. (requires a PR to be created first)
+7. Follow step 4 and onwards from the [General](#General) guide above to bring it to the finish line.
+
+### Pull Requests
+
+Please try to keep your pull request focused in scope and avoid including unrelated commits.
+
+After you have submitted your pull request, we'll try to get back to you as soon as possible. We may suggest some changes or request improvements, therefore, please check ✅ ["Allow edits from maintainers"](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork) on your PR.
+
+## Valtio-specific Guideline
+
+##### Documentation
+
+1. Navigate to the [`website`](./website/) folder. (e.g., `cd website`).
+2. Run `pnpm install` to install dependencies in the `website` folder.
+3. Run `pnpm run dev` to start the dev server.
+4. Navigate to [`http://localhost:3000`](http://localhost:3000) to view the documents.
+5. Navigate to the [`docs`](./docs/) folder and make necessary changes to the documents.
+6. Add your changes to the documents and see them live reloaded in the browser.
+7. Follow step 4 and onwards from the [General](#General) guide above to bring it to the finish line.
+
+Thank you for contributing! :heart: