Ui documentation update

Add info on javascript and css libraries in code style section of docs.

docs/code_style.md

@@ -30,3 +30,8 @@ The quickest way to correct import style locally is to let isort make the change
 Note: It's not possible to disable isort wrapping style checking, so for now we've chosen the
 most deterministic [wrapping mode](https://github.com/timothycrosley/isort#multi-line-output-modes)
 to reduce the line length guess-work when adding imports, even though it's not the most concise.
+
+UI
+--
+
+We use the [Airbnb](https://github.com/airbnb/javascript) style guide for Javascript and validate it with ESlint (see Validating Javascript in the [UI development section](ui/installation.md#validating-javaScript)). For CSS, we use [reactstrap](https://reactstrap.github.io/) and Bootstrap's utility classes as much as possible before adding custom CSS to a style sheet. Any custom style that can be made reusable should be named generically and stored in the ``ui/css/treeherder-global.css`` file.