Add example for os.environ.setdefault() and envs interpolation

examples/providers/configuration/config-with-env-var.yml

@@ -0,0 +1,2 @@
+section:
+  option: ${ENV_VAR}

examples/providers/configuration/configuration_env_interpolation_os_default.py

@@ -0,0 +1,19 @@
+"""`Configuration` provider values loading example."""
+
+import os
+
+from dependency_injector import containers, providers
+
+
+class Container(containers.DeclarativeContainer):
+
+    config = providers.Configuration()
+
+
+if __name__ == '__main__':
+    os.environ.setdefault('ENV_VAR', 'default value')
+
+    container = Container()
+    container.config.from_yaml('config-with-env-var.yml')
+
+    assert container.config.section.option() == 'default value'