add contributing guide

Author: scniro

README.md

@@ -181,5 +181,15 @@ render() {
 
 Check out [bokuweb/re-resizable](https://github.com/bokuweb/re-resizable). Wrapping your component with `<Resizable/>'s` works well
 
+## Contributing
+
+Pull Requests are welcome. Be mindful of the availbe scripts below to help submitting a well-received contribution.
+
+- `npm run start` to run the app on `localhost:8000`
+- `npm run webpack` to re-bundle the react app
+- `npm run test` to ensure tests continue to pass
+- `npm run build` to generate the demo bundle
+
+* note that it's necessary to bump the [package.json](https://github.com/scniro/react-codemirror2/blob/master/package.json#L3) version prior to final `npm run build` so we can grab the proposed new version as seen in the demo header. Also note, the core changes are to be made in `src/index.tsx` as `./index.js` and `./index.d.ts` are _generated_
 
 [MIT](./LICENSE) © 2018 [scniro](https://github.com/scniro)