Add autoprefixer to PostCSS installation examples

Author: reinink

docs/source/docs/installation.blade.md

@@ -94,6 +94,7 @@ module.exports = {
   plugins: [
     // ...
     tailwindcss('./path/to/your/tailwind-config.js'),
+    require('autoprefixer'),
     // ...
   ]
 }
@@ -113,6 +114,7 @@ gulp.task('css', function () {
         .pipe(postcss([
           // ...
           tailwindcss('./path/to/your/tailwind-config.js'),
+          require('autoprefixer'),
           // ...
         ]))
         // ...