slight update to readme

README.rst

-big field and particle simulator
+Big Field and Particle Simulator
+================================
+
+At the moment, this code is meant to run pseudospectral DNS of
+Navier-Stokes, using FFTW 3, and to integrate particle trajectories in
+the resulting fields.
+I'm trying to write it as general as possible, so that it can be
+expanded in the future; it remains to be seen how well this will work.