cmake tweak: set the HDF5_ROOT environment variable if necessary

It turns out that FindHDF5 searches the HDF5_ROOT environment variable
rather than the equivalent cmake variable. So if the user specifies the
cmake variable, then we simply set the same environment variable before
calling find_package.
Updated README with build instructions
Updated readme with install section
Updated in-repo HDF5 SWMR build

This is a static build. Use with cmake -DHDF5_USE_STATIC_LIBRARIES=ON
travis tweak: updated to latest cmake conf