Update README
Update README to mirror below: (Feedback from CEA)
- the README does not give requirements, we saw that json-devel and catch2 (for tests) are required;
- is the requirement for the json-devel version a huge one? the package available on RHEL8 is not recent enough (3.6.1 vs 3.9.1);
- depending on g++ version, linking with 'stdc++fs' is mandatory to use the filesystem library (as said on the Notes section here: https://en.cppreference.com/w/cpp/filesystem);
- the README states that we can use the options '-DHESTIA_INSTALL_XXX' to configure the install paths, I think those are '-DCMAKE_INSTALL_XXX' instead;