docs: document app cp command syntax

See coop-cloud/organising#245.

cli/app/cp.go

@@ -21,6 +21,18 @@ var appCpCommand = &cli.Command{
 	Aliases:   []string{"c"},
 	ArgsUsage: "<src> <dst>",
 	Usage:     "Copy files to/from a running app service",
+	Description: `
+This command supports copying files to and from any app service file system.
+
+If you want to copy a myfile.txt to the root of the app service:
+
+    abra app cp <app> myfile.txt app:/
+
+And if you want to copy that file back to your current working directory locally:
+
+		abra app cp <app> app:/myfile.txt .
+
+`,
 	Action: func(c *cli.Context) error {
 		app := internal.ValidateApp(c)