Add documentation for new @psalm-private-set annotation

docs/annotating_code/supported_annotations.md

@@ -324,6 +324,27 @@ echo Arithmetic::addCumulative(3); // outputs 3
 echo Arithmetic::addCumulative(3); // outputs 6
 ```
 
+### `@psalm-private-set`
+
+Used to annotate properties which can only be set in a private context. With this, public properties can be read from another class but only be modified within a method of its own class.
+
+```php
+class Counter {
+  /** @psalm-private-set */  
+  public int $count = 0;
+    
+  public function increment() : void {
+    $this->count++;
+  }
+}
+
+$counter = new Counter();
+echo $counter->count; // outputs 0
+$counter->increment(); // Method can modify property
+echo $counter->count; // outputs 1
+$counter->count = 5; // This will fail as it's modifying a property directly
+```
+
 ## Type Syntax
 
 Psalm supports PHPDoc’s [type syntax](https://docs.phpdoc.org/guides/types.html), and also the [proposed PHPDoc PSR type syntax](https://github.com/php-fig/fig-standards/blob/master/proposed/phpdoc.md#appendix-a-types).