add to readme that tailwind.config.js is required

fix #74

Author: ledenis

README.md

@@ -48,3 +48,7 @@ Before:
 After:
 
 <img src="https://raw.githubusercontent.com/bradlc/vscode-tailwindcss/master/img/css-highlighting-after.png" alt="CSS syntax highlighting after" width="400">
+
+## Requirements
+
+This extension requires a `tailwind.config.js` file to be [present in your project folder](https://github.com/bradlc/vscode-tailwindcss/blob/master/package.json#L26). You can create it with `npx tailwind init`.