docs: fix fontweight

[zoepage]

http://hoodiehq.github.io/hoodie-css/colour-fonts.html

we use 300, 400 && 500. please fix this is the documentation :)

[verpixelt]

I'd like to put the font-weight: 400 up for discussion. We're using it just in a few places where imho either 300 || 500 would be a fit. This could save us a few kb in the font.css file.

[tamacodechi]

I'd like to weigh in ;)
I think it's fine to take out 400. Poked around changing it up for 300 and 500 and nothing drastic happened. It would save a nice relative amount of kb.
(Maybe add to the docs how to add 400 if someone using the hoodie-css for their own thing wants to - maybe even on a guide/docs for the basic template so there's not too much clutter?)

[NickColley]

👍 for removing 400, fonts are so huge :(