Add some info to README.md

[lsawade]

Hi!

I think it would be helpful for users to explicitly state that the HDF5 environment variables have to be set, e.g., HDF5DIR, LD_LIBRARY_PATH addition, etc.

In retrospective it seems obvious, but it would be nice to add as an FYI if Parallel HDF5 is installed manually and not loaded from cluster modules. For the manual case, it would also help the user to know, if HDF5 is installed manually, that it has to be installed with the following flags:

./configure --enable-shared --enable-parallel \
    --enable-fortran --enable-fortran2003 \
    --prefix=<path/to/your/build>

Thanks!

Cheers!

[lsawade]

This commit should fix most issue under which the standard installation procedure didn't work:

https://github.com/SeismicData/asdf-library/commit/f28...

Should we maybe add to the readme that it's easiest to compile by explicitly naming the c and fortran compilers, h5pcc and h5pfc, respectively? It already shows it as a last resort, but why go to the last resort if you have to define com compiler names anyways?