add i2lua documentation

contrib/i2lua/README.md

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+i2lua is an experimental i2p router with an embedded lua interpreter for use in test networks and research.
+
+For now, don't use this unless you know exactly what you want to do, how to do it and want to hack on the source code.
+
+Building:
+
+    git clone https://github.com/majestrate/kovri -b development kovir-dev
+    mkdir build
+    cd build
+    cmake -DWITH_LUA=ON -DWITH_AESNI=ON ../kovri-dev
+    make -j8
+
+Running:
+
+    ./i2lua example.lua

contrib/i2lua/example.lua

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+--
+-- run i2p router with no clients on port 50050
+--
+
+print("Starting i2lua")
+
+-- initialize i2lua router to ise port 50050
+port = 50050
+print("Using port",port)
+i2lua.Init(port)
+
+-- start the router up
+i2lua.Start()
+print("Router Up")
+
+-- wait for the router to stop
+-- the program will wait here or until SIGINT or a crash
+i2lua.Wait()
+print("Exiting...")