Update README

README.txt

@@ -177,7 +177,6 @@ Downloading from Repositories
         |         |
       nuttx/     apps/
 
-
   Initialize Sub-Modules
 
     The NuttX repository contains GIT sub-modules for the Documentation/,
@@ -188,6 +187,11 @@ Downloading from Repositories
       git submodule init
       git submodule update
 
+    This command can be used later to refresh all submodules with the
+    latest files:
+
+      git submodule foreach git pull
+
   Configuring the Clones
 
     The following steps need to be performed for each of the two