Update README with abstract & attribution

Author: drsteve

README.md

@@ -1,13 +1,35 @@
 # PTM - The SHIELDS Particle Tracing Model
+The SHIELDS Particle Tracing Model (PTM) is a large-scale particle tracing code, primarily intended for magnetospheric
+applications. It provides a fast, parallel computing-friendly, and versatile framework for tracing charged particles in arbitrary
+electric and magnetic fields. To support a broad range of test particle applications, full Lorentz motion can be modeled as
+well as guiding center motion. Switching strategies allow accurate and efficient test particle tracing across regions where
+the guiding center approximation may not be maintained. PTM supports tracing of particle motion both forwards and
+backwards in time. For backwards tracing, PTM can launch particles from a specified location and trace until stopping
+criteria are met. These stopping criteria can include crossing a surface in space, or a time limit. For forwards tracing
+particles can be launched from specified points or spatial regions. The code is designed to allow many applications without
+requiring recompilation, instead providing a rich, user-configurable interface that allows use of different modes at runtime.
+Model results are stored in user-selected format for post-processing including Liouville mapping of particle fluxes from
+source to target location. PTM includes pre-processing for field configurations, tools for setting up runs, and postprocessing
+capabilities to enable scientific use. Sample applications include specifying the geosynchronous particle
+boundary for inner magnetosphere modeling, and determining access of solar energetic protons.
 
-### Authors and Contributors
-Jesse Woodroffe [1], Steven Morley [1], Thiago Brito [1], Alin-Daniel Panaitescu [2], Michael Henderson [1], Vania Jordanova [1]
+This code is approved for open source release by Los Alamos National Laboratory and has been assigned reference number C21002.
+
+### Attribution
+The following people have contributed to the edvelopment of SHIELDS-PTM:
+Jesse Woodroffe [1], Steven Morley [1], Thiago Brito [1], Alin-Daniel Panaitescu [2], Miles Engel [1], Michael Henderson [1], Vania Jordanova [1]
 
 1. ISR-1, Space Science & Applications, Los Alamos National Laboratory
 2. ISR-2, Space and Remote Sensing, Los Alamos National Laboratory
 
+When using this code, please acknowledge its use, refer the reader to the GitHub repository and cite the master DOI at Zenodo.
+A paper describing the code is forthcoming, but until this is updated the best references for this code are:
+- [Woodroffe et al., Data-optimized source modeling with the Backwards Liouville Test-Kinetic method, JASTP, 2018](https://doi.org/10.1016/j.jastp.2017.09.010)
+- [Jordanova et al., Specification of the near-Earth space environment with SHIELDS, JASTP, 2018](https://doi.org/10.1016/j.jastp.2017.11.006)
+- [Brito et al., Particle tracing modeling of ion fluxes at geosynchronous orbit, JASTP, 2018](https://doi.org/10.1016/j.jastp.2017.10.008)
+
 ### About the code
-This directory contains the simulation code and analysis tools for the SHIELDS particle tracing model (PTM).
+This repository contains the simulation code and analysis tools for the SHIELDS particle tracing model (PTM).
 
 The main source is written in Fortran2008. Associated scripts, pre-processing, and post-processing are written in Python 3.
 
@@ -19,6 +41,6 @@ make
 To build the fortran only run `make ptm`, to build/install the Python module only run `make python`, and to convert the Markdown documents to PDF use `make docs`.
 The documentation conversion uses the Node.js `mdpdf` module, which can be installed using `npm install mdpdf -g`. PDF documentation is placed in the `docs` directory.
 
-### RKSUITE
+### Use of RKSUITE
 In addition to the basic RK4 integrator, SHIELDS-PTM provides an interface to the RKSUITE library allowing users to select higher-order, adaptive integrators.
 If these integrators are used, RKSUITE and its originators should be credited. For details, please see [rksuite_readme](src/rksuite_readme).