Added reference to content block mixins

The rationale behind this PR is that when we have multi-line mixins, they usually have a context change from the default flow of rules for a particular selector: placing before the 'default' rules makes it harder to see what's happening with a quick glance.

See the changed bit of the readme [here](https://github.com/HouseTrip/css-style-guide/tree/content-blocks-order#ordering-properties).

PS: Sorry for the whitespace removal, Sublime is really aggressive with it :)