Add a comment explaining the .env file (#200)

See:
https://devcenter.heroku.com/articles/heroku-local#set-up-your-local-environment-variables

Author: edmorley

.env

@@ -1 +1,6 @@
+# This file specifies the environment variables that will be set when using the
+# `heroku local` command locally during development. It has no effect on apps
+# running on Heroku - to set env vars for those, see:
+# https://devcenter.heroku.com/articles/config-vars
+
 TIMES=2