add docs for aggragation function fixes #501 (#510)

* add docs for aggragation function fixes #501

* add missing reference fixes #501

doc/features/aggregation.rst

@@ -0,0 +1,44 @@
+.. _aggregation:
+
+Aggregation
+===========
+
+``min()`` returns the smallest of the given Money objects
+
+.. code-block:: php
+
+    $first = Money::EUR(100);                  // €1.00
+    $second = Money::EUR(200);                 // €2.00
+    $third = Money::EUR(300);                  // €3.00
+
+    $min = Money::min($first, $second, $third) // €1.00
+
+``max()`` returns the largest of the given Money objects
+
+.. code-block:: php
+
+    $first = Money::EUR(100);                  // €1.00
+    $second = Money::EUR(200);                 // €2.00
+    $third = Money::EUR(300);                  // €3.00
+
+    $max = Money::max($first, $second, $third) // €3.00
+
+``avg()`` returns the average value of the given Money objects as a Money object
+
+.. code-block:: php
+
+    $first = Money::EUR(100);                  // €1.00
+    $second = Money::EUR(-200);                // -€2.00
+    $third = Money::EUR(300);                  // €3.00
+
+    $avg = Money::avg($first, $second, $third) // €2.00
+
+``sum()`` provides the sum of all given Money objects
+
+.. code-block:: php
+
+    $first = Money::EUR(100);                  // €1.00
+    $second = Money::EUR(-200);                // -€2.00
+    $third = Money::EUR(300);                  // €3.00
+
+    $sum = Money::sum($first, $second, $third) // €2.00

doc/index.rst

@@ -47,6 +47,7 @@ subtle intricacies of handling money.
    features/allocation
    features/parsing
    features/formatting
+   features/aggregation
 
 .. toctree::
    :hidden: