docs: add whitelist section to purge (#250)

* docs: add whitelist section to purge

* Update tailwind-config.md

* Update tailwind-config.md

Co-authored-by: Sébastien Chopin <seb@nuxtjs.com>

docs/content/en/tailwind-config.md

@@ -9,7 +9,7 @@ category: Guide
 
 ## Default Configuration
 
-```js
+```js{}[tailwind.config.js]
 {
   theme: {},
   variants: {},
@@ -79,7 +79,7 @@ Learn more about the [Tailwind config](https://tailwindcss.com/docs/configuratio
 
 You can also use your `nuxt.config.js` to set your Tailwind Config with the `tailwindcss.config` property:
 
-```js{}[nuxt.config.js]
+```js [nuxt.config.js]
 import tailwindTypography from '@tailwindcss/typography'
 
 export default {
@@ -186,6 +186,22 @@ This merging strategy of with a function only applies to `plugins` and `purge.co
 
 </alert>
 
+### Whitelisting classes
+
+If you need to whitelisting classes and avoid purge system, you need to precise `safelist` option like below :
+
+```js{}[tailwind.config.js]
+module.exports = {
+  purge: {
+    options: {
+      // Whitelisting some classes to avoid purge
+      safelist: ['whitelisted']
+    }
+  }
+}
+```
+
+
 ## Referencing in the application
 
 It can often be useful to reference tailwind configuration values in runtime. For example to access some of your theme values when dynamically applying inline styles in a component.