docs(readme): add link for logo postcss

Author: Scrum

Diff for: readme.md

@@ -1,4 +1,4 @@
-# postcss-at-rules-variables plugin for <img valign="middle" height="49" title="PostCSS" src="http://postcss.github.io/postcss/logo.svg">
+# postcss-at-rules-variables plugin for <a href="https://github.com/postcss/postcss"><img valign="middle" height="49" title="PostCSS" src="http://postcss.github.io/postcss/logo.svg"></a>
 > [PostCSS](https://github.com/postcss/postcss) plugin to transform [W3C CSS Custom Properties](http://www.w3.org/TR/css-variables/) for at-rule’s parameters.
 
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