Add description of upstream repo

externals/json/README.md

@@ -0,0 +1,7 @@
+JSON for Modern C++
+===================
+
+This is a mirror providing the single required header file.
+
+The original repository can be found at
+ https://github.com/nlohmann/json/commit/21d23982cafe826cae088b2bb37cb40c8e7a81fe