Add superbuild instructions in SMTK README.md
current build tutorial.
It would be nice if we can mention the superbuild in the top level readme.md since it's the simplest way to build smtk.
Edited by Haocheng LIU
current build tutorial.
It would be nice if we can mention the superbuild in the top level readme.md since it's the simplest way to build smtk.