Add SHUMlib installation instructions for non-MO sites

[yaswant]

Shumlib currently builds using pure makefiles and installation at the Met Office is managed using a script that calls make with the appropriate arguments. Other sites have provided their own set of make macros to define flags for their sites.

Now that shumlib is being used by LFRic atm and LFRic inputs, the installation process would benefit from being streamlined to make it site-neutral and to allow it to be installed and managed by spack.