[WIKI] Using less mixins from extension stylesheets.

[zaggino]

https://github.com/adobe/brackets/wiki/How%20to%20write%20extensions

Load a CSS file: use ExtensionUtils.loadStyleSheet()

is missing an information, how to use brackets mixins (e.g. brackets_mixins.less) in extension stylesheet. It'd be quite handy to be able to use this, so I don't have to redefine mixins in my own less stylesheet.

[gruehle]

Reviewed. To @peterflynn to make sure this is capture in the extension API updates.