Add Info On Setup To README.md

[rtillery]

Setting up use of fmtster and {fmt} isn't exactly difficult, but it should be outlined in the README.

• Prerequisites (gcc & {fmt} obviously, but what else?)
• How to install {fmt} (source & repo)
• How to build fmtstertest
• How to run fmtstertest

[rtillery]

Comment from @Alexcornila

• for fmtster to work with fmt requires that fmt is compiled as a dynamic library (.so); set the flag
  cmake -DBUILD_SHARED_LIBS=TRUE ..
• build fmtster with make
• tell the os where to locate library at run time
  echo $LD_LIBRARY_PATH
  LD_LIBRARY_PATH=/path/to/lib.so
  export LD_LIBRARY_PATH
  ./example-json

[rtillery]

• for fmtster to work with fmt requires that fmt is compiled as a dynamic library (.so); set the flag

fmtster does not require {fmt} to be built as a dynamic library. Only if you wish to build the example and test applications is that a consideration. Note that those apps are not needed to use fmtster. It's just a header. Your apps can be built with static or dynamic {fmt}.

• build fmtster with make

make does not build fmtster. It only builds the example and test applications.
fmtster does not need to be built, as it is just a header.

• tell the os where to locate library at run time
  echo $LD_LIBRARY_PATH
  LD_LIBRARY_PATH=/path/to/lib.so

This will replace any existing library path with just the {fmt} library path, losing any previous value. LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/path/to/lib.so is a better choice, to maintain whatever paths were previously configured. (but see below)

export LD_LIBRARY_PATH

This will not persistent across terminal loads. You would need to add this to a shell initialization script (e.g. ~/.profile)

However, this is not the best way to complete the installation. It is better to place the library in the existing system path. The fmt documentation indicates that sudo make install will handle this. Then no modification of LD_LIBRARY_PATH is required.

Note that if you build {fmt} with the defaults, it will build and install a static library (libfmt.a) in your library path. If you have issues building an app (e.g. the examples or tests here) and switch to a dynamic library (adding the CMake flag mentioned above), it will also be built and installed in your library path. However, when you try to build the app, both libraries will be in the library path. The linker will give precedence to the static library and the build will fail again. The static library will need to be manually removed from the build path. On Ubuntu 20.04, the libraries are installed in /usr/lib.