docs: add yarn installation (#73)

yarn add eslint-plugin-security --dev

Author: Hashen110

README.md

@@ -6,7 +6,7 @@ This project will help identify potential security hotspots, but finds a lot of
 
 ### Installation
 
-`npm install --save-dev eslint-plugin-security`
+`npm install --save-dev eslint-plugin-security` or `yarn add eslint-plugin-security --dev`
 
 ### Usage