docs: Added Set::delete() documentation

Author: rudashi

docs/Set.md

@@ -6,6 +6,7 @@ The Set object holds only unique values of any type.
 
 - [`Set() constructor`](#new-set)
 - [`Set::add()`](#setadd)
+- [`Set::delete()`](#setdelete)
 - [`Set::has()`](#sethas)
 
 ## Constructor
@@ -52,6 +53,20 @@ $set->add(40)->add('second');
 // [40, 'first', 'second']
 ```
 
+### Set::delete()
+
+The `delete()` method removes a specified value from the Set.
+
+```php
+$set = new Set(['foo', 'bar']);
+
+$set->delete('foo');
+// true
+
+$set->delete(1);
+// false
+```
+
 ### Set::has()
 
 The `has()` method returns `TRUE` when element with the specified value exists in the `Set` or `FALSE` if it does not exist.