Docs should mention that layout styles support percentage values
Hey all 👋 I'm the maintainer of [html-sketchapp](https://github.com/brainly/html-sketchapp) and I'm currently rebuilding the project so that it uses react-sketchapp for rendering (instead of rendering directly to Sketch).

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| Requesting a new feature
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**Proposal/Feature-request:**
It looks like react-sketchapp supports both point and percentage values in the layout styles (width, top, margin, etc) just like [react native](https://facebook.github.io/react-native/docs/layout-props.html#width) does. However, it is not clear from the [react-sketchapp docs](http://airbnb.io/react-sketchapp/docs/styling.html) which ATM only mention that these values should be numbers. IMO it'd be valuable to mention that percentages are also available e.g. by updating "type" of these props to `number | string`, `points | percentages` or defining a helper type (just like `Color`).
