doc: update readme with installtion (#2)

Author: marcosvega91

README.md

@@ -49,4 +49,8 @@ Actually the plugin works for `html` language and `jsx` language.
 
 ## Install
 
-N/A
+Install prettier-plugin-tailwind-css:
+
+```
+npm install --save-dev prettier-plugin-tailwind-css
+```