DOC: Noted that default-constructed smart pointers in main() are null
DOC: Noted that default-constructed smart pointers in main() are null

Added note that default-constructed smart pointers locally declared within `main(argc, argv)` are initialized to null, as was suggested by @mstaring at pull request #94.

Follow-up to commit 49421b181302c75e22158e282254c30b44631b55

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix