docs: add CONTRIBUTING.md (#34)

CONTRIBUTING.md

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+# SWR DevTools Contributing Guidelines
+
+Thank you for your contributions!
+
+## Local Development
+
+### Develop `swr-devtools-panel`
+
+```
+$ yarn install
+$ yarn start
+```
+
+And then, open http://localhost:3000/.
+
+### Develop SWR DevTools as Web Extension
+
+#### Chrome
+
+After the instruction of "Develop `swr-devtools-panel`", open chrome://extensions/ on Chrome and select the `dist/` directory from the `Load unpacked` button.
+See more details on https://developer.chrome.com/docs/extensions/mv3/getstarted/.
+
+#### Firefox
+
+After the instruction of "Develop `swr-devtools-panel`", run `yarn workspace swr-devtools-extensions start:firefox`.