docs: add patron button

Author: shellscape

README.md

@@ -31,11 +31,11 @@ Using npm:
 npm install postcss-less --save-dev
 ```
 
-Using yarn:
+<a href="https://www.patreon.com/shellscape">
+  <img src="https://c5.patreon.com/external/logo/become_a_patron_button@2x.png" width="160">
+</a>
 
-```console
-yarn add postcss-less --dev
-```
+Please consider [becoming a patron](https://www.patreon.com/shellscape) if you find this module useful.
 
 ## Usage