Add upgrade instructions (#81)

Co-authored-by: Alexander Makarov <sam@rmcreative.ru>

UPGRADE.md

@@ -0,0 +1,42 @@
+# Upgrading Instructions for Yii Widget 
+
+### !!!IMPORTANT!!!
+
+The following upgrading instructions are cumulative. That is,
+if you want to upgrade from version A to version C and there is
+version B between A and C, you need to following the instructions
+for both A and B.
+
+## Upgrade from 1.x
+
+* `beforeRun`, `afterRun` and `run` methods have been removed from the `Widget` class, and only the `render` method 
+  can now be used, so you may need to make some changes. For example:
+  ```php
+  protected function beforeRun(): bool
+  {
+      //...
+      return true;
+  }
+
+  protected function afterRun(string $result): string
+  {
+      //...
+      return $result;
+  }
+
+  protected function run(): string
+  {
+       //...
+       return $content;
+  }
+  ```
+  will be:
+  ```php
+  public function render(): string|\Stringable
+  {
+      // "beforeRun" stuff
+      // "run" stuff
+      // "afterRun" stuff
+      return $result;
+  }
+  ```