Updated readme to provide more context

Author: JanDC

README.md

@@ -1,2 +1,9 @@
 # page-specific-css
-Php library which determines critical css on a per page basis.
+Page-specific-css is a library with extracts css selectors from html snippets and matches them with css rules to extract the used rules.
+
+This effectively enables you to create critical css.
+
+The bulk of it's code is based on, and expanded from, Thijs Verkoyen's inline css library.
+
+### Usage
+This library is used as a twig post processing module in JanDC/critical-css-processor. But you could of course make your own implementation.