[mc] Adding MODPACK download option similar to WORLD

minecraft-server/README.md

@@ -301,6 +301,17 @@ will be deleted when the container is deleted.
 you should use an IP address or a globally resolveable FQDN, or else the
 name of a linked container.
 
+### Downloadable mod pack
+
+Like the `WORLD` option above, you can specify the URL of a "mod pack"
+to download and install into the `mods` directory. To use this option
+pass the environment variable `MODPACK`, such as
+
+    docker run -d -e MODPACK=http://www.example.com/mods/modpack.zip ...
+
+**NOTE:** The referenced URL must be a zip file with one or more Forge jar files at the
+top level of the zip archive.
+
 ## JVM Configuration
 
 ### Memory Limit