Improve README with optional parameters

Author: fmassa

README.md

@@ -62,6 +62,14 @@ optnet = require 'optnet'
 optnet.optimizeMemory(net, input, opts)
 ```
 
+### Optional parameters
+
+Here is a list of options that are currently supported, and should be passed in the `opts` table as a third argument:
+* `inplace`: uses in place modules when available (boolean)
+* `mode`: selects between `training` and `evaluation` optimization algorithm (string)
+* `reuseBuffers`: shares internal buffers between same modules (like unfolded images for convolution) (boolean)
+* `removeGradParams`: remove `gradWeight` and `gradBias` in the networks, saving their sharings so that they can be exactly reconstructed. Only applies for `evaluation` mode (boolean)
+
 ## Visualizing the memory reuse
 
 We can analyse the sharing of the internal buffers by looking at the computation