Readme reference to font picker feature

[brynwhyman]

Now that this module includes features to update theme frontend colours and fonts, we should update the readme to reference these and provide user/developer documentation.

• Reference to theme colour picker (Update readme.md #20)
• Reference to font picker

[robbieaverill]

We have this already: https://github.com/silverstripe/cwp-agencyextensions/blob/master/docs/en/01_Features/ThemeColors.md

Feel free to reopen if you think the docs are lacking =)

[brynwhyman]

I missed this... I can't find anything on the font picker so reopening