NFD+ci+docs: Attaching NFD as a submodule

Based on NFD:commit:27be0b0d41235609bee703f4f2ef9e5b1978fdce (version 0.2.0-74-g27be0b0)

Change-Id: I7f9117b226aa244e257375f7f4c045dc9c4cd897
Refs: #3124
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..d01dda7
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "NFD"]
+	path = NFD
+	url = https://github.com/named-data-ndnSIM/NFD