[Openexr-user] Adding a note about installation procedure to downloads page

[David Doria]

I think on this page:
http://www.openexr.com/downloads.html

There should be a note at the very top or somewhere else visible like
"You must first build and install IlmBase before you can build
OpenEXR. You will get errors about missing files (half.h, etc) if you
do not".

David