add readme file adapted from...

add readme file adapted from https://github.com/pmla/polyhedral-template-matching/blob/master/README.md to USER-PTM package

src/USER-PTM/README

+
+This package provides the compute ptm/atom command for LAMMPS which implements the
+Polyhedral Template Matching algorithm for analysis of molecular dynamics simulation data
+
+The method is currently (Fall 2018) included in OVITO, LAMMPS, and ASAP.
+
+OVITO PTM documentation: http://www.ovito.org/manual/particles.modifiers.polyhedral_template_matching.html
+
+LAMMPS PTM documentation: http://lammps.sandia.gov/doc/compute_ptm_atom.html
+
+ASAP PTM documentation: http://wiki.fysik.dtu.dk/asap/Local%20crystalline%20order
+
+
+If you use PTM in a publication, please reference:
+
+Peter Mahler Larsen and Søren Schmidt and Jakob Schiøtz
+Robust structural identification via polyhedral template matching
+Modelling Simul. Mater. Sci. Eng. 24 (2016) 055007
+
+Article available at:
+http://stacks.iop.org/0965-0393/24/i=5/a=055007
+
+Preprint available at:
+http://arxiv.org/abs/1603.05143
+
+