[TASK] Mention styleguide extension in README (#332)

Author: cweiske

README.md

@@ -5,3 +5,8 @@ SVG icons for the TYPO3 CMS backend
 ## Style guide
 
 Website: https://typo3.github.io/TYPO3.Icons/
+
+## Alternatives
+
+To see all icons available in your TYPO3 installation, install the official
+[styleguide extension](https://github.com/TYPO3-CMS/styleguide).