update README

README.rst

+================================
 Big Fluid and Particle Simulator
 ================================
 
@@ -12,6 +13,32 @@ in the repository), and it is working as expected. Parameters and
 statistics are stored in HDF5 format, together with code information,
 so simulation data should be "future proof".
 
+Installation
+------------
+
+**Postprocessing only**
+
+.. code:: bash
+    python setup.py install
+
+(add `--user` or `sudo` as appropriate).
+`setup.py` should tell you about the various packages you need.
+
+**Full installation**
+
+If you want to run simulations on the machine where you're installing,
+you will need to call `build` before installing.
+
+.. code:: bash
+    python setup.py build
+    python setup.py install
+
+The `build` command will most likely fail unless you modify
+`machine_settings.py` appropriately for your machine.
+Also, in order to run the C++ code you need to have an MPI compiler
+installed, the HDF5 C library as well as FFTW 3 (at least 3.3 I think).
+
+
 Comments
 --------
 

bfps/cpp/rFFTW_interpolator.cpp

@@ -24,6 +24,8 @@
 
 
 
+#define NDEBUG
+
 #include "rFFTW_interpolator.hpp"
 
 template <class rnumber, int interp_neighbours>