Added installation instructions for Arch Linux

I have compiled a short installation guide for Arch-based distributions. As for MPI implementation, I followed your choice of MPICH, but if ease of installation is to be preferred over consistency across distributions, it may be better to go with [OpenMP](https://www.archlinux.org/packages/?q=openmp), which is available as a binary package. I can test that combination on Arch, if you want.

Although the configuration script didn't complain, I'm not sure about octave and mhash, because I couldn't find any -dev packages. If you tell me how to test if the packages are sufficient for Kaira, I will do that.

I hope I don't have any typos in the package names.