Please document the cmake build instructions

[rouson]

Most importantly, it's not clear how to build all of the tests. My steps were

git clone git@github.com:jacobwilliams/json-fortran
mkdir -p json-fortran/build
cd json-fortran/build
cmake ..
make -j 8

after which I was able to build each individual test in a manner such as

make jf_test_01

but I couldn't find instructions in the README.md or on the wiki for building all of the tests at once. I tried make test and make tests to no avail.

[jacobwilliams]

make check is what you want I believe. This should build the unit tests and run them.

I can update the documentation. Note: I usually use FoBiS to build them (e.g., FoBiS.py build -f json-fortran.fobis -mode tests-gnu).