Update README.md

[askourtis]

README.md needs to be updated.

For example it states the following usage MPIManager(;np=Sys.CPU_THREADS, mpi_cmd=false, launch_timeout=60.0) which is wrong.

• v0.1.0 has an optional parameter mpirun_cmd. Constructor signature on v0.1.0
• v0.2.0 does not offer such parameter. Constructor signature on v0.2.0

[vchuravy]

Happy to take a PR.