ndnsec: add a short description to the `--help` output of each command

Change-Id: Ifcc217bb096f1f27bceb609f5f117cf8b1d0c73d
diff --git a/tools/ndnsec/sign-req.cpp b/tools/ndnsec/sign-req.cpp
index 2023ce9..f3f714f 100644
--- a/tools/ndnsec/sign-req.cpp
+++ b/tools/ndnsec/sign-req.cpp
@@ -37,6 +37,8 @@
   po::options_description description(
     "Usage: ndnsec sign-req [-h] [-k] [-n] NAME\n"
     "\n"
+    "Generate a certificate signing request.\n"
+    "\n"
     "Options");
   description.add_options()
     ("help,h", "produce help message")