I created a `nlohmann-json` package for `build2` (https://build2.org) that have been published on the central public repository https://cppget.org : https://cppget.org/nlohmann-json (at the moment you can see that there is only one version: 3.7.3).

The repository of the package code is there: https://github.com/build2-packaging/nlohmann-json and is owned by the build2 community now. (with build2 you can use either a repository like cppget.org or a git repository so both are useful to users for different use cases).

This patch just add the basic information as to how to use this library with build2.