Removes old README and replaces with markdown version

README.md

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+SW4 - Seismic Waves, 4th order accuracy
+===========================================================
+[![License GPL2+:](https://img.shields.io/badge/License-GPL%202%2B-red)](https://github.com/geodynamics/sw4/blob/master/LICENSE.txt)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.8322590.svg)](https://doi.org/10.5281/zenodo.8322590)
+[![pdf manual](https://img.shields.io/badge/get-PDF-green.svg)](https://github.com/geodynamics/sw4/blob/master/doc/SW4_UsersGuide.pdf)
+
+More information
+----------------
+Please see the [Users Guide](https://github.com/geodynamics/sw4/blob/master/doc/SW4_UsersGuide.pdf) for more information regarding the use of SW4.
+
+Additional resources:
+- [SW4](https://geodynamics.org/resources/sw4/about)
+- [Forum](https://community.geodynamics.org/c/sw4/32)
+
+
+Files
+-----
+
+- LICENCE.txt -    GNU General Public Licence version 2
+- INSTALL.txt  - Information on how to build SW4
+- README.txt  - This file!
+- wave.txt    - Text file containing the "SW4 Lives" banner
+- Makefile     - Main makefile
+
+Directories
+-----------
+- configs/    -  Directory containing configuration files for "make"
+- src/        -  C++ and Fortran source code for SW4
+- tools/       - Matlab/Octave scripts for post processing and analysis of results
+- examples/   - Sample SW4 input files
+- optimize/    - Directory for object files and the optimized SW4 executable
+- debug/       - Directory for object files and a SW4 executable with debug symbols
+
+License
+-------
+
+SW4 is published under [GPL v2 or newer](LICENSE.txt).
+
+