add particles

Author: zingale

paper/paper.md

@@ -75,7 +75,9 @@ The current pyro solvers are:
 
   * shallow water equations solver
 
-(solvers since the first pyro paper are marked with a ☆).
+(solvers since the first pyro paper are marked with a ☆).  Also
+new is support for Lagrangian tracer particles, which can be added to
+any solver that has a velocity field.
 
 
 # References