Update README for PHP 8 compatibility fork

Added a disclaimer about the forked repository and updated the installation command to reflect the new package name compatible with PHP 8. This ensures clarity for users on the origin and purpose of the changes.

src/Jlinn/Mandrill/Struct.php

@@ -7,6 +7,9 @@
 
 namespace Jlinn\Mandrill;
 
+use ArrayIterator;
+use Traversable;
+
 abstract class Struct implements \IteratorAggregate{
     /**
      * Create a Struct from an associative array
@@ -44,11 +47,12 @@ public function toArray(){
      * (PHP 5 &gt;= 5.0.0)<br/>
      * Retrieve an external iterator
      * @link http://php.net/manual/en/iteratoraggregate.getiterator.php
-     * @return \Traversable An instance of an object implementing <b>Iterator</b> or
+     * @return Traversable|ArrayIterator An instance of an object implementing <b>Iterator</b> or
      * <b>Traversable</b>
      */
-    public function getIterator(){
-        return new \ArrayIterator($this->toArray());
+    public function getIterator(): \Traversable|\ArrayIterator
+    {
+        return new ArrayIterator($this->toArray());
     }