add attributions file to credit open source contributions

ATTRIBUTIONS.md

+Open source software doesn't happen in a vaccum. NestMC prototype includes code from the open source community. Thank you!
+
+## JSON for Modern C++
+
+A header only C++11 json library, written by Niels Lohmann (github user nlohmann).
+MIT license.
+
+https://github.com/nlohmann/json
+
+## Templatized C++ Command Line Parser Library (TCLAP)
+
+A header only C++ library for command line argument parsing, written by Michael E. Smoot.
+MIT license
+
+https://sourceforge.net/projects/tclap
+