Update README with required software

The required software in the README is not correct (e.g. an MPI library is not required), and should be updated with the actual information.

Also need to update how to install the package, and get the ibtracs data: ftp://ftp.gfdl.noaa.gov/perm/GFDL_pubrelease/TSTORMS/tstorm_ibtracs_data.tar.gz