Tutorial for the image readers and the LoadImage transform

[wyli]

As the experimental new APIs have been implemented for MONAI I/O Project-MONAI/MONAI#909
this ticket looks for a tutorial to show that:

• the image reader APIs could be used independently as file format specific loaders
• LoadImage transform could be used as a format-agnostic module, typically as the first transform in a 'transform chain'

(might be useful to briefly mention the optional import feature of MONAI)

[wyli]

fixed by #19