Add docs for environment varibles interpolation in the Configuration provider

Author: rmk135

docs/providers/configuration.rst

@@ -30,6 +30,10 @@ where ``examples/providers/configuration/config.ini`` is:
  :language: ini
  :linenos:
 
+:py:meth:`Configuration.from_ini` supports environment variables interpolation. Use
+``${ENV_NAME}`` format in the configuration file to substitute value of environment
+variable ``ENV_NAME``.
+
 Loading from ``yaml`` file
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -48,6 +52,10 @@ where ``examples/providers/configuration/config.yml`` is:
  :language: ini
  :linenos:
 
+:py:meth:`Configuration.from_yaml` supports environment variables interpolation. Use
+``${ENV_NAME}`` format in the configuration file to substitute value of environment
+variable ``ENV_NAME``.
+
 .. note::
 
  Loading configuration from yaml requires ``PyYAML`` package. You can install