Added dynamic severity example (#758)

Author: suhaibasaeed

allure-pytest/examples/label/severity/dynamic_severity.rst

@@ -0,0 +1,10 @@
+Dynamic Severity
+-------------
+
+    >>> import allure
+    >>> import pytest
+
+    >>> def test_dynamic_severity():
+    ...     # Some condition
+    ...     if True:
+    ...       allure.dynamic.severity(allure.severity_level.CRITICAL)