Update README.md with PRIF 0.4 citation

[bonachea]

Need to update README.md with PRIF 0.4 citation:

Dan Bonachea, Katherine Rasmussen, Brad Richardson, Damian Rouson.
"[**Parallel Runtime Interface for Fortran (PRIF) Specification, Revision 0.4**](https://github.com/BerkeleyLab/caffeine/wiki/pubs/PRIF_0.4.pdf)",
Lawrence Berkeley National Laboratory Technical Report (LBNL-2001604), July 2024.
<https://doi.org/10.25344/S4WG64>

I'm thinking this should happen not before prif.f90 has been updated with the appropriate interface declarations.

[bonachea]

Fixed in c8b9d42f