Add docs for side effect feature on spies

Author: inderpreet99

CHANGELOG.rst

@@ -1,3 +1,9 @@
+1.13.0
+------
+
+* The object returned by ``mocker.spy`` now also tracks any side effect
+  of the spied method/function.
+
 1.12.1 (2019-11-20)
 -------------------
 

README.rst

@@ -103,6 +103,9 @@ features with it, like retrieving call count. It also works for class and static
 Since version ``1.11``, it is also possible to query the ``return_value`` attribute
 to observe what the spied function/method returned.
 
+Since version ``1.13``, it is also possible to query the ``side_effect`` attribute
+to observe any exception thrown by the spied function/method.
+
 Stub
 ----