Update docs

Author: rmk135

docs/providers/configuration.rst

@@ -43,8 +43,18 @@ where ``examples/providers/configuration/config.ini`` is:
    :language: ini
 
 :py:meth:`Configuration.from_ini` method supports environment variables interpolation. Use
-``${ENV_NAME}`` format in the configuration file to substitute value of the environment
-variable ``ENV_NAME``.
+``${ENV_NAME}`` format in the configuration file to substitute value from ``ENV_NAME`` environment
+variable.
+
+You can also specify a default value using ``${ENV_NAME:default}`` format. If environment
+variable ``ENV_NAME`` is undefined, configuration provider will substitute value ``default``.
+
+.. code-block:: ini
+
+   [section]
+   option1 = {$ENV_VAR}
+   option2 = {$ENV_VAR}/path
+   option3 = {$ENV_VAR:default}
 
 Loading from a YAML file
 ------------------------
@@ -62,12 +72,22 @@ where ``examples/providers/configuration/config.yml`` is:
 .. literalinclude:: ../../examples/providers/configuration/config.yml
    :language: ini
 
-:py:meth:`Configuration.from_yaml` method uses custom version of ``yaml.SafeLoader``.
+:py:meth:`Configuration.from_yaml` method supports environment variables interpolation. Use
+``${ENV_NAME}`` format in the configuration file to substitute value from ``ENV_NAME`` environment
+variable.
 
-The loader supports environment variables interpolation. Use ``${ENV_NAME}`` format
-in the configuration file to substitute value of the environment variable ``ENV_NAME``.
+You can also specify a default value using ``${ENV_NAME:default}`` format. If environment
+variable ``ENV_NAME`` is undefined, configuration provider will substitute value ``default``.
 
-You can also specify a YAML loader as an argument:
+.. code-block:: ini
+
+   section:
+     option1: {$ENV_VAR}
+     option2: {$ENV_VAR}/path
+     option3: {$ENV_VAR:default}
+
+:py:meth:`Configuration.from_yaml` method uses custom version of ``yaml.SafeLoader``.
+To use another loader use ``loader`` argument:
 
 .. code-block:: python