Write documentation about adding custom styles

[gavinwye]

Problem

When building prototypes designers often 'hack' styles quickly to get what they want. Sometimes these hacks make their way into production prototypes. There are a number of reasons that this is a bad idea, although prototyping should be quick and dirty so wqe need to allow this.

Solution

1. Add a _hacks.scss and import it into @import
2. Update the documentation for the HMRC kit to explain this

Link to original slack chat for @gavinwye
https://hmrcdigital.slack.com/archives/int-team-ui/p1470048104000008

[feedmypixel]

This has the benefit of also providing an example of how to @import if you wish to import your own file. As discussed this line is currently a little confusing.