Fix variable not set warning

Added an untracked `.env` file to the root directory to prevent the
`WARNING: The INSTALL_PRESTISSIMO variable is not set` message from being
displayed when `docker-compose` is run.

Added section in README for optional features.

docker-compose.yml

@@ -156,7 +156,7 @@ services:
         build:
             context: ./workspace
             args:
-                INSTALL_PRESTISSIMO: ${INSTALL_PRESTISSIMO}
+                - INSTALL_PRESTISSIMO=${INSTALL_PRESTISSIMO}
         volumes_from:
             - application
         tty: true