Updating arg docs in the proper place

Signed-off-by: Jarrod Pooler <jarrod.pooler@homesite.com>

docs/compose-file.md

@@ -119,6 +119,29 @@ Add build arguments. You can use either an array or a dictionary. Any
 boolean values; true, false, yes, no, need to be enclosed in quotes to ensure
 they are not converted to True or False by the YML parser.
 
+First, specify the arguments in your Dockerfile:
+
+    ARG buildno
+    ARG password
+
+    RUN echo "Build number: $buildno"
+    RUN script-requiring-password.sh "$password"
+
+Then specify the arguments under the `build` key. You can pass either a mapping
+or a list:
+
+    build:
+      context: .
+      args:
+        buildno: 1
+        password: secret
+
+    build:
+      context: .
+      args:
+        - buildno=1
+        - password=secret
+
 Build arguments with only a key are resolved to their environment value on the
 machine Compose is running on.