Wrote part of configuration chapter

device-config
bridging-unicast
xconnect
pseudowire

Change-Id: Ib393d52e7bdc606c705e1e14dd52c9fcd77c0755
diff --git a/configuration/external-connectivity.rst b/configuration/external-connectivity.rst
new file mode 100644
index 0000000..fe91e21
--- /dev/null
+++ b/configuration/external-connectivity.rst
@@ -0,0 +1,2 @@
+External Connectivity
+=====================