Add linker instruction for local execution

[BenWiederhake]

If the user doesn't call make install, the executables can't easily find the shared library. One way to fix this is using LD_LIBRARY_PATH=path/to/the/so/ ./flif Documents #432

[jonsneyers]

Thanks, but should no longer be necessary because of https://github.com/FLIF-hub/FLIF/pull/436