docs: add upgrade guide

Author: yajra

UPGADE.md

@@ -0,0 +1,22 @@
+# UPGRADE GUIDE
+
+## Upgrade from 10.x to 11.x
+
+1. Update the composer.json file and change the version of the package to `^11.0`:
+
+```json
+"require": {
+    "yajra/laravel-datatables-buttons": "^11.0"
+}
+```
+
+2. Run `composer update` to update the package.
+
+3. If you are using a custom class of laravel-excel to export the data, you need to update the collection method and include the return type `Collection|LazyCollection` in the method signature.
+
+```php
+public function collection(): Collection|LazyCollection
+{
+    return $this->collection;
+}
+```