docs(contributing): Document use of editorconfig

CONTRIBUTING.md

@@ -17,6 +17,10 @@ Get in touch with us through twitter or via the [Stryker gitter](https://gitter.
 * [@_simondel](https://twitter.com/_simondel)
 * [@_nicojs](https://twitter.com/_nicojs)
 
+## Code style
+
+Please adher to our [editorconfig](https://editorconfig.org) rules. The easiest way to do that is by installing the editorconfig plugin in your code editor of choice. For VSCode, use the [editorconfig vscode extension](https://github.com/editorconfig/editorconfig-vscode#editorconfig-for-visual-studio-code)
+
 ## Runner Stryker locally
 
 We use [Lerna](https://lernajs.io/) to manage the packages in this repository. You don't have to install it globally. The packages themselves can be found in the [packages folder](https://github.com/stryker-mutator/stryker/tree/master/packages). npm commands such as `npm test` can be executed from the root of the project but executing them inside of a package folder is more time efficient. However, we suggest running `npm test` in the root of the project before a commit to ensure that everything still works. To get Stryker running locally, please follow these steps: