Make note of required arguments

Author: rjhenry

README.md

@@ -94,6 +94,14 @@ docker run -d --net bookstack_nw \
 The APP_URL parameter should be the base URL for your BookStack instance without a trailing slash. For example:
 APP_URL=http://example.com
 
+The following environment variables are required for Bookstack to start:
+- `APP_KEY`
+- `APP_URL`
+- `DB_HOST` (in the form `${hostname_or_ip_address}:${port}`)
+- `DB_DATABASE`
+- `DB_USERNAME`
+- `DB_PASSWORD`
+
 #### Volumes
 To access your `.env` file and important bookstack folders on your host system change `<HOST>` in the following line to your host directory and add it then to your run command: