IJulia demonstration notebook  -- updated based on @vtjnash comment

Amit, Thanks for creating LibExpat.jl.  I have been finding it very useful.  This pull request would add a demonstration IJulia Notebook along with a hyperlink to nbviewer from README.md.    If you pull it, you'll probably want to change the nbviewer URL in README.md to replace 'catawbasam' with 'amitmurphy'.