Add code of conduct and contributing files (reduxjs#395)

* add code of conduct

* add contributing guide

* update building section on contributing

* format files

* update stack overflow tag on contributing

* improve submitting changes section on contributing

CODE_OF_CONDUCT.md

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+# Contributor Code of Conduct
+
+As contributors and maintainers of this project, we pledge to respect all people who contribute through reporting issues, posting feature requests, updating documentation, submitting pull requests or patches, and other activities.
+
+We are committed to making participation in this project a harassment-free experience for everyone, regardless of level of experience, gender, gender identity and expression, sexual orientation, disability, personal appearance, body size, race, age, or religion.
+
+Examples of unacceptable behavior by participants include the use of sexual language or imagery, derogatory comments or personal attacks, trolling, public or private harassment, insults, or other unprofessional conduct.
+
+Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct. Project maintainers who do not follow the Code of Conduct may be removed from the project team.
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by opening an issue or contacting one or more of the project maintainers.
+
+This Code of Conduct is adapted from the [Contributor Covenant](http://contributor-covenant.org), version 1.0.0, available at [http://contributor-covenant.org/version/1/0/0/](http://contributor-covenant.org/version/1/0/0/)

CONTRIBUTING.md

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+# Contributing
+
+We are open to, and grateful for, any contributions made by the community. By contributing to Redux Toolkit, you agree to abide by the [code of conduct](https://github.com/reduxjs/redux-toolkit/blob/master/CODE_OF_CONDUCT.md).
+
+## Reporting Issues and Asking Questions
+
+Before opening an issue, please search the [issue tracker](https://github.com/reduxjs/redux-toolkit/issues) to make sure your issue hasn't already been reported.
+
+Please ask any general and implementation specific questions on [Stack Overflow with a Redux Toolkit tag](http://stackoverflow.com/questions/tagged/redux-toolkit?sort=votes&pageSize=50) for support.
+
+## Development
+
+Visit the [Issue tracker](https://github.com/reduxjs/redux-toolkit/issues) to find a list of open issues that need attention.
+
+Fork, then clone the repo:
+
+```
+git clone https://github.com/your-username/redux-toolkit.git
+```
+
+### Building
+
+Running the `build` task will create both a CommonJS module-per-module build and a UMD build.
+
+```
+npm run build
+```
+
+### Testing and Linting
+
+To run the tests:
+
+```
+npm run test
+```
+
+To continuously watch and run tests, run the following:
+
+```
+npm test -- --watch
+```
+
+To perform linting with `eslint`, run the following:
+
+```
+npm run lint
+```
+
+### New Features
+
+Please open an issue with a proposal for a new feature or refactoring before starting on the work. We don't want you to waste your efforts on a pull request that we won't want to accept.
+
+## Submitting Changes
+
+- Open a new issue in the [Issue tracker](https://github.com/reduxjs/redux-toolkit/issues).
+- Fork the repo.
+- Create a new feature branch based off the `master` branch.
+- Make sure all tests pass and there are no linting errors.
+- Submit a pull request, referencing any issues it addresses.
+- If you changed external-facing types, make sure to also build the project locally and include the updated API report file etc/redux-toolkit.api.md in your pull request.
+
+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 improvements.
+
+Thank you for contributing!