document --e, --env command line flag

docs/source/faq.md

@@ -24,3 +24,9 @@ or similar traditional docker command.
 
 Check out the [binder-examples](http://github.com/binder-examples/) github
 organization for example Dockerfiles you can copy & modify for your own use!
+
+## How do I set environment variables?
+
+Use the `-e` or `--env` flag for each variable that you want to define.
+
+For example `jupyter-repo2docker -e VAR1=val1 -e VAR2=val2 ...` 

docs/source/usage.rst

@@ -131,6 +131,14 @@ be used by docker directly.
 
      jupyter-repo2docker --no-build --debug https://github.com/jakevdp/PythonDataScienceHandbook
 
+Setting environment variables
+-----------------------------
+
+If you want to define environment variables, you can pass the ``--env`` or ``-e`` parameter to the commandline:
+
+  .. code-block:: bash
+
+     jupyter-repo2docker -e VAR1=val1 -e VAR2=val2 ...
 
 Accessing help from the command line
 ------------------------------------