README tweak

Author: fmassa

README.md

@@ -60,3 +60,36 @@ g = generateGraph(net, input)
 graph.dot(g,modelname..'_optimized',modelname..'_optimized')
 ```
 ![GoogleNet with memory optimization](doc/googlenet_optimized.gif)
+
+## Counting the amount of saved memory
+
+We can also provide a function to compute the amount of memory used by the network
+in bytes, which allows us to check the amount of saved memory. It currently
+counts only the `output` fields of each module, and not it's internal buffers.
+
+Here is an example
+
+```lua
+optnet = require 'optnet'
+utils = require 'optnet.utils'
+usedMemory = utils.usedMemory
+
+models = require 'optnet.models'
+modelname = 'googlenet'
+net, input = models[modelname]()
+
+mem1 = usedMemory(net, input)
+
+optnet.optimizeMemory(net, input)
+
+mem2 = usedMemory(net, input)
+
+optnet.removeOptimization(net)
+
+mem3 = usedMemory(net, input)
+
+print('Before optimization        : '.. mem1/1024/1024 .. ' MBytes')
+print('After optimization         : '.. mem2/1024/1024 .. ' MBytes')
+print('After removing optimization: '.. mem3/1024/1024 .. ' MBytes')
+
+```