Info about building via git is incomplete

[aradi]

I just tried to follow the instructions on https://docs.lfortran.org/en/installation/ for building LFortran from the Git-repo.

If I do it, the compiler builds, but when I try to compile a source file with it, I get an error message about missing clang compiler:

sh: clang: command not found                                                     
The command 'clang -o hello.out hello.tmp.o  -L"/mnt/scratch/aradi/BUILD/lfortran
/src/bin/../runtime" -Wl,-rpath,"/mnt/scratch/aradi/BUILD/lfortran/src/bin/../run
time" -llfortran_runtime -lm' failed.

I think, one should either declare in the documentation, that a clang compiler is needed (not just any working C++ compiler), or, even better, add clang as an additional to be installed dependency when creating the conda environment.

Note: This is the same issue as reported for the conda-forge recipe at conda-forge/lfortran-feedstock#110