Update readme.md

This package contains a bias potential that can be used to study solid-liquid transitions with the interface pinning method.

This is done by adding a harmonic potential to the Hamiltonian that bias the system towards two-phase configurations. 

This example demonstrate using a bias potential that can be used to study solid-liquid transitions with the interface pinning method. This is done by adding a harmonic potential to the Hamiltonian that bias the system towards two-phase configurations. 

  U_bias = 0.5*k*(Q-a)^2

  urp.dk/interface_pinning.htm 

for a detailed bibliography. 

# Build

Remember to include the following command when building LAMMPS

  make yes-user-pinning

# Use

   fix [name] [groupID] rhok [nx] [ny] [nz] [spring-constant] [anchor-point]

  Ulf R. Pedersen

  http://www.urp.dk

  ulf AT urp.dk

# Cite

Please cite 

  [Ulf R. Pedersen, J. Chem. Phys. 139, 104102 (2013)]