Add sponsor

Author: jgthms

README.md

@@ -6,4 +6,8 @@
 
 The content of this project itself is licensed under the [Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-nc-sa/4.0/).
 
-The underlying source code used to format and display that content is licensed under the [MIT license](http://opensource.org/licenses/mit-license.php).
+The underlying source code used to format and display that content is licensed under the [MIT license](http://opensource.org/licenses/mit-license.php).
+
+# Sponsor
+
+<a target='_blank' rel='nofollow' href='https://app.codesponsor.io/link/5pucRKHwhsqiAiYS1CdnS4SM/jgthms/css-reference'>  <img alt='Sponsor' width='888' height='68' src='https://app.codesponsor.io/embed/5pucRKHwhsqiAiYS1CdnS4SM/jgthms/css-reference.svg' /></a>