Explain memory usage of Overdrive offline phases.

README.md

@@ -166,3 +166,7 @@ These programs can be run similarly to `spdz2-offline.x`, for example:
 `host2:$ ./simple-offline.x -p 1 -h host1`
 
 Running any program without arguments describes all command-line arguments.
+
+##### Memory usage
+
+Homomorphic ciphertexts are relatively large (in the order of megabytes), and the zero-knowledge proofs we use require storing some hundred of them. You must therefore expect to use at least some hundred megabytes of memory per thread. The memory usage is linear in `MAX_MOD_SZ` (determining the maximum integer size for computations in steps of 64 bits), so you can try to reduce it (see the compilation section for how set it). For some choices of parameters, 4 is enough while others require up to 8. The programs above indicate the minimum `MAX_MOD_SZ` required, and thez fail during the parameter generation if it is too low.