Add README for third-party dependency descriptions

Author: trflynn89

extern/README.md

@@ -0,0 +1,15 @@
+# External Libraries
+
+This directory contains all third-party libraries used by libfly for testing purposes only. The core
+libfly library does not have any depedencies (except for the STL). The libraries here are only for
+unit testing and benchmarking comparisons.
+
+Libraries used for unit testing:
+
+* [Catch2](https://github.com/catchorg/Catch2)
+
+Libraries used for benchmarking:
+
+* [{fmt}](https://github.com/fmtlib/fmt)
+* [Boost.JSON](https://github.com/boostorg/json)
+* [JSON for Modern C++](https://github.com/nlohmann/json)