ndn-cxx+build+docs: Attaching ndn-cxx as a submodule

Based on ndn-cxx:commit:6fa52970f35a58df85dae9c1cca7be6c58ab7d0a (version 0.3.4)

Change-Id: I1f3e5bfcd09c7dd517862ccaf6abe169cf71ecd4
Refs: #3124
diff --git a/.gitmodules b/.gitmodules
index d01dda7..eda145d 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,6 @@
 [submodule "NFD"]
 	path = NFD
 	url = https://github.com/named-data-ndnSIM/NFD
+[submodule "ndn-cxx"]
+	path = ndn-cxx
+	url = https://github.com/named-data-ndnSIM/ndn-cxx