Update README according to postcss/postcss-custom-properties#1 & post…

…css/postcss-custom-properties#9

plugins/postcss-custom-properties/README.md

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 > [PostCSS](https://github.com/postcss/postcss) plugin to transform [W3C CSS Custom Properties for cascading variables](http://www.w3.org/TR/css-variables/) syntax to more compatible CSS.
 
-**N.B.** For now the transformation _is not complete_. It currently just aims to provide a future-proof way of using a _limited subset (to `:root` selector)_ of the features provided by native CSS custom properties.  
-
-_[Checkout opened issue to know the state of this plugin](https://github.com/postcss/postcss-custom-properties/issues)._
+**N.B.** The transformation _is not complete_. It currently just aims to provide a future-proof way of using a **limited subset (to `:root` selector)** of the features provided by native CSS custom properties. Read [#1](https://github.com/postcss/postcss-custom-properties/issues/1) & [#9](https://github.com/postcss/postcss-custom-properties/issues/9) to know why this limitation exists.
 
 Works great with [postcss-calc](https://github.com/postcss/postcss-calc).