Added architecture and design

Also added a note about
  - Reserved MPLS label
  - Reserved VLAN id
  - Reserved multicast address

Change-Id: I86e0c8ed37622bf383ec3d615956f4e0b34f91be
diff --git a/configuration/device-config.rst b/configuration/device-config.rst
index 4337558..6da9acd 100644
--- a/configuration/device-config.rst
+++ b/configuration/device-config.rst
@@ -42,6 +42,10 @@
     - When using OpenvSwitch, set this to ``ovs-ofdpa``
     - When using hardware switches, set this to ``ofdpa3`` or eliminate ``driver`` entirely as the correct driver configuration will be set automatically by ONOS
 
+.. caution::
+    We should avoid using reserved MPLS labels for ``ipv4NodeSid`` and ``ipv6NodeSid``.
+    Please check here for the reserved values: http://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml
+
 .. note::
     Most of the Trellis configurations support dynamic configuration updates.
     Unfortunately, Trellis currently **do not support dynamic device configuration updates**.