added JS documentation to the contributing section

docs/contributing/index.rst

@@ -7,3 +7,4 @@ Contributing to Wagtail
 
     developing
     css_guidelines
+    js_guidelines

docs/contributing/javascript_guidelines.rst

@@ -0,0 +1,51 @@
+JavaScript coding guidelines
+============================
+
+Write JavaScript according to the `Airbnb Styleguide <http://github.com/airbnb/javascript>`_, with some exceptions:
+
+-  Use soft-tabs with a four space indent. Spaces are the only way to
+   guarantee code renders the same in any person's environment.
+-  We accept ``snake_case`` in object properties, such as
+   ``ajaxResponse.page_title``, however camelCase or UPPER_CASE should be used
+   everywhere else.
+
+
+Linting and formatting code
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Wagtail provides some tooling configuration to help check your code meets the
+styleguide. You'll need node.js and npm on your development machine.
+Ensure project dependencies are installed by running ``npm install``
+
+
+**Linting code**
+
+``npm run lint:js``
+
+This will lint all the JS in the wagtail project, excluding vendor
+files and compiled libraries.
+
+Some of the modals are generated via server-side scripts. These include
+template tags that upset the linter, so modal workflow JavaScript is
+excluded from the linter.
+
+
+**Formatting code**
+
+``npm run format:js``
+
+This will perform safe edits to conform your JS code to the styleguide.
+It won't touch the line-length, or convert quotemarks from double to single.
+
+Run the linter after you've formatted the code to see what manual fixes
+you need to make to the codebase.
+
+**Changing the linter configuration**
+
+Under the hood, the tasks use the `JavaScript Code Style <http://jscs.info/>`_ library.
+
+To edit the settings for ignored files, or to change the linting rules,
+edit the ``.jscsrc`` file in the wagtail project root.
+
+A complete list of the possible linting rules can be found here:
+`JSCS Rules <http://jscs.info/rules.html>`_