Add C usage to documentation.
Add C usage to documentation.

Fixes #114.
luics/css-layout,facebook/css-layout,yihuang/css-layout,yihuang/css-layout,rmarinho/yoga,pragmatrix/css-layout,yihuang/css-layout,yihuang/css-layout,rmarinho/yoga,luics/css-layout,yihuang/css-layout,facebook/css-layout,gethinw/css-layout,yihuang/css-layout,rmarinho/yoga,emilsjolander/css-layout,facebook/yoga,gethinw/css-layout,gethinw/css-layout,emilsjolander/css-layout,yihuang/css-layout,rmarinho/yoga,yihuang/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,luics/css-layout,facebook/css-layout,rmarinho/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/css-layout,emilsjolander/css-layout,facebook/css-layout,rmarinho/yoga,rmarinho/yoga,yihuang/css-layout,rmarinho/yoga,facebook/yoga,rmarinho/yoga,luics/css-layout,rmarinho/yoga,emilsjolander/css-layout,facebook/yoga,yihuang/css-layout,pragmatrix/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,pragmatrix/css-layout,facebook/yoga,pragmatrix/css-layout,gethinw/css-layout