docs+build: Fix build on Ubuntu 16.04

python-sphinx on Ubuntu 16.04 requires `master_doc` to exist, otherwise
failing the build of manpages, even though this `master_doc` is never
used in manpages.

Change-Id: I8252cd8d24cfcc0403fdf551519944692e206380
diff --git a/manpages/conf.py b/manpages/conf.py
index d86777a..913f0c8 100644
--- a/manpages/conf.py
+++ b/manpages/conf.py
@@ -1,6 +1,8 @@
 # General information about the project.
 project = u'NDN Essential Tools'
 
+master_doc = 'index'
+
 # -- Options for manual page output ---------------------------------------
 
 # One entry per manual page. List of tuples
diff --git a/manpages/index.rst b/manpages/index.rst
new file mode 100644
index 0000000..d76db17
--- /dev/null
+++ b/manpages/index.rst
@@ -0,0 +1,8 @@
+.. toctree::
+  ndn-dissect
+  ndn-pib
+  ndndump
+  ndnpeek
+  ndnping
+  ndnpingserver
+  ndnpoke