Add a note about custom properties

Close postcss#3

Author: MoOx

README.md

@@ -95,6 +95,11 @@ whatever {
 
 No we cannot do that. `currentcolor` depends on the cascade (so the DOM) and we can't handle that in a simple preprocessing step. You need to handle that with polyfills.
 
+### Can we use CSS custom properties so we can do `color(var(--mainColor) adjuster())`?
+
+By using [postcss-custom-properties](https://github.com/postcss/postcss-custom-properties) before this plugin, you can do that (sort of).
+You have some examples in [cssnext playground](http://cssnext.github.io/cssnext-playground/).
+
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 ## Contributing