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.

.env

@@ -0,0 +1 @@
+INSTALL_PRESTISSIMO=false

README.md

@@ -49,6 +49,7 @@ It's like Laravel Homestead but for Docker instead of Vagrant.
 		- [Run a Docker Virtual Host](#Run-Docker-Virtual-Host)
 		- [Find your Docker IP Address](#Find-Docker-IP-Address)
 		- [Use custom Domain](#Use-custom-Domain)
+		- [Optional Features](#Optional-Features)
 		- [Debugging](#debugging)
 - [Help & Questions](#Help)
 
@@ -822,6 +823,13 @@ server_name laravel.dev;
 ```
 
 
+<br>
+<a name="Optional-Features"></a>
+### Optional Features
+#### prestissimo
+[prestissimo](https://github.com/hirak/prestissimo) is a plugin for composer which enables parallel install functionality. You can enable prestissimo by setting `INSTALL_PRESTISSIMO=true` in the `.env` file in the Laradock directory.
+
+
 <br>
 <a name="debugging"></a>
 ### Debugging

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