Added readme file, and removed old files

examples/USER/pinning/readme.md

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+This bias potential is used to study solid-liquid transitions with the interface pinning method.
+
+Reference:
+
+[Ulf R. Pedersen, J. Chem. Phys. 139, 104102 (2013)] 
+
+Usage snip:
+
+   fix [fix-name] [groupID] rhoKUmbrella [nx] [ny] [nz] [kappa] [anchor-point]
+   thermo_style custom step temp pzz pe lz f_umbrella f_umbrella[1] f_umbrella[2] f_umbrella[3]
+
+where the parameters set the harmonic bias potential U=0.5*kappa*(|rho_k|-anchor-point)^2 
+with the wave-vector elements of rho_k to k_x = (2 pi / L_x) * n_x, k_y = (2 pi / L_y) * n_y and k_z = (2 pi / L_z) * n_z.
+
+This package was created by
+  Ulf R. Pedersen
+  http://www.urp.dk
+  ulf AT urp.dk