chore: add contributing guidelines (#129)

Author: lynnmercier

CONTRIBUTING.md

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+
+# Contributing
+
+MDC React is an open source project that accepts contributions from community members.
+
+## Code of Conduct
+
+The MDC React authors value respect and are committed to making this repository a safe space for all peoples.
+
+To learn more about what to expect from us and what we expect from you, read the [Code of Conduct](https://github.com/material-components/material-components/blob/develop/CODE_OF_CONDUCT.md).
+
+
+## Issues and Bugs
+
+If you find a bug in the source code or a mistake in the documentation, you can help us by submitting an issue.
+
+Even better: propose a fix with a pull request and link it to the issue!
+
+
+## Feature or Component Requests
+
+If you want to request a new component, or change to the UX of an existing component, you must first make this request in our [MDC Web repository](https://github.com/material-components/material-components-web/issues/new). Please read the general [Material Design Components Request Policy](https://github.com/material-components/material-components/blob/develop/COMPONENTS_REQUEST_POLICY.md) before you request any changes.
+
+Once the issue is resolved and the component built, we will update our [roadmap](ROADMAP.md) with a timeline that includes wrapping the MDC Web component in MDC React.
+
+
+## Pull Requests
+
+Please only submit small pull requests, less than 200 lines of modification. MDC React is built by wrapping MDC Web code, so fixes should be isolated to this small wrapping layer of code.
+
+If you think you need a much larger pull request, like an architecture change, then first open an issue detailing your proposed architecture. We will discuss the proposal on the issue thread before we review any code in a pull request.
+
+
+### Code review
+
+All code goes through code review before it can be merged into main branches.
+