helper+docs+examples: New helper to simplify link failing

Thanks to Saran Tarnoi for providing base implementation of the code
diff --git a/examples/wscript b/examples/wscript
index 437ed26..49a0c5c 100644
--- a/examples/wscript
+++ b/examples/wscript
@@ -91,3 +91,6 @@
         'ndn-simple-api.cc',
         'custom-apps/ndn-api-app.cc'
         ]
+
+    obj = bld.create_ns3_program('ndn-simple-with-link-failure', all_modules)
+    obj.source = 'ndn-simple-with-link-failure.cc'