Add reference to "installation documentation"

[cponder]

This page here https://github.com/NCAR/ParallelIO#Building-with-CMake has the statement

Full instructions for the cmake build can be found in the installation documentation.

I think you mean this page here https://ncar.github.io/ParallelIO/install.html I had to find it by googling. Can you add the link to the original page there?

[jedwards4b]

completed