add info about eslint

Author: etpinard

CONTRIBUTING.md

@@ -116,8 +116,9 @@ To view the results of a run on CircleCI, download the `build/test_images/` and
 
 ### Coding style
 
-- 4-space indentation
-- semi-colons are required
-- trailing commas
-
 Check if ok, with `npm run lint`
+
+- See [eslintrc](https://github.com/plotly/plotly.js/blob/master/.eslintrc) and
+  the eslint [list of rules](http://eslint.org/docs/rules/) for more details.
+- Rules listed in the eslintrc file with the ignore flag `0` are the recommended
+  rules for new code added.