README: clarify make-ring init command

This patch explicits what the make-ring --init command really does as
it might be confusing to use.

Change-Id: I0044aee3733d18ebb45c2b7c00a1b073f9a33336
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

README.rst

@@ -22,6 +22,15 @@ Initialize the repositories
 
     ./make-ring.py --init
 
+It initializes and updates the submodules to set them at the top of their master branch. This
+is ideal to have the latest development version.
+
+However, in order to build a specific version of Ring, such as the Production one, please use
+
+.. code-block:: bash
+
+    git submodule update --init
+
 On Linux
 --------
 

docs/source/dev/compiling_and_installing/ring-project.rst

@@ -13,6 +13,15 @@ Initialize the repositories
 
     ./make-ring.py --init
 
+It initializes and updates the submodules to set them at the top of their master branch. This
+is ideal to have the latest development version.
+
+However, in order to build a specific version of Ring, such as the Production one, please use
+
+.. code-block:: bash
+
+    git submodule update --init
+
 On Linux
 ########