Add documenation about env variables to Mix module.

Author: Daniel Perez

lib/mix/lib/mix.ex

@@ -152,6 +152,24 @@ defmodule Mix do
   with extra clean up logic.
 
   Note aliases do not show up on `mix help`.
+
+  ## Environment variables
+
+  Environment variables can be used to modify Mix behaviour.
+
+  Mix responds to the following variables:
+
+    * `MIX_ARCHIVE` - allows specifying the directory into which the archives should be installed
+    * `MIX_DEBUG`   - outputs debug information about each task before running it
+    * `MIX_ENV`     - allows specifying which environment should be used. see Environments
+    * `MIX_EXS`     - allows changing the full path to the `mix.exs` file
+    * `MIX_HOME`    - stores configuration files and scripts shared by multiple implementations
+    * `MIX_PATH`    - allows expanding the code path
+    * `MIX_QUIET`   - does not print information messages to the terminal
+
+  Variables which do not take a value should be set to either `1` or `true`, for example:
+
+      $ MIX_DEBUG=1 mix compile
   """
 
   use Application