Add notebook for shadow tomography

[fdmalone]

@wjhuggings.

It would be nice to have as it's probably something people are most interested in.

[fdmalone]

xref #346

[mhucka]

@fdmalone It's not clear (to me) from the comment above – has this issue been addressed, by any chance? Should this be closed?

[fdmalone]

Can probably close it, it wasn't addressed, but it would be @wjhuggins to add (can't seem to assign him)

[mhucka]

Thanks for the quick reply. I'll go ahead and close it. If it's something people want, this can be reopened in the future.