fix typo in README

The README says:
> "The library itself contains of a single header file ..."

I guess "consists of a" is the wording that was intended here. Alternatively, "contains a" might also be what was intended. Either way, I think both versions work equally well and thus choose the one that I believe was intended.

Fix typo

Misc cmake packaging enhancements

CMake convention is to use a project namespace, i.e. Foo::, for imported
targets.  When multiple targets are imported from a project, this looks
like Foo::Bar1 Foo::Bar2, etc.  This adds the nlohmann_json:: namespace to
the exported target names.

This also allows the generated project config files to be used from the
build directory instead of just the install directory.

Fixed incorrect LLVM version number in README

See https://blog.travis-ci.com/2018-04-09-say-bonjour-to-xcode-93-and-macos-high-sierra.

clang-900.0.39.2 is from LLVM 9.0.0, not 8.1.0

Fix trivial typo in comment.

Not really important, but since I saw it.

Package Manager: Spack

I contributed a package to the Spack package manager.

Spack is a flexible package manager that supports multiple versions,
configurations, platforms, and compilers. It is popular in
high-performance computing.

CMake: 3.8+ is Sufficient

The current CMake scripts depend on CMake 3.8+.
This allows us to remove previous work-arounds.

Added public target_compile_features for auto and constexpr