docs: add contributing guide

Fixes #246

PiperOrigin-RevId: 591967524

Author: asyncLiz

CONTRIBUTING.md

@@ -0,0 +1,59 @@
+# Contributing
+
+Thank you for your interest in contributing! The following sections describe
+ways to get involved.
+
+## Code of conduct
+
+Please review and follow our [code of conduct](CODE_OF_CONDUCT.md).
+
+## Feedback
+
+User feedback is the most valuable to us. It's a great way to start
+contributing!
+
+-   [File new issues](https://github.com/material-components/material-web/issues/new/choose)
+    for bugs you run into or feature requests you have.
+
+-   [Create a discussion](https://github.com/material-components/material-web/discussions/new/choose)
+    for help, feedback on changes, or feature proposals.
+
+## Discord
+
+Join the `#material` channel on [Lit's Discord](https://lit.dev/discord) to chat
+directly with the team and other users.
+
+## Pull requests
+
+Pull requests are welcome! Keep a few things in mind:
+
+-   Create an
+    [issue](https://github.com/material-components/material-web/issues/new/choose)
+    or
+    [discussion](https://github.com/material-components/material-web/discussions/new/choose)
+    before opening a pull request.
+-   Trivial changes, such as documentation, don't need an issue.
+-   Create draft PRs in the `@material/web/labs` folder for new features.
+-   Please be patient! It may take a while for the team to review. Keep changes
+    small and scoped to speed things up.
+
+### New components
+
+Please understand that new component implementations are difficult for us to
+directly accept. They need to complete several engineer, design, and
+accessibility reviews that are not easy with our externally available workflows.
+
+If you want to help build a new component, create a
+[new discussion](https://github.com/material-components/material-web/discussions/new/choose).
+Add any design docs, code samples, reference implementations in draft PRs, and
+get community feedback.
+
+### Contributor License Agreement
+
+Code contributions must
+[sign Google's CLA](https://cla.developers.google.com/clas). When you open a
+pull request, our friendly bot will check and provide help if you haven't
+signed.
+
+[Set your email in git](https://help.github.com/articles/setting-your-email-in-git/)
+to the same email used to sign the CLA.