add a photutils tutorial

[kelle]

started by @JenniferKarr and @bsipocz

[bsipocz]

@kelle - I've added a getting started tutorial for photutils in #99, please have a look when you have a chance.

[kelle]

another place to get inspired for photutils is from STScI: http://stsci-riab.github.io/riatraining/photometry.html

[bsipocz]

There are a bunch of notebooks using photutils here: https://github.com/astropy/photutils-datasets/tree/master/notebooks

[adrn]

Note that @bsipocz started a tutorial in #99, but this would still be a good place for someone to contribute!

[keflavich]

https://github.com/spacetelescope/notebooks/pull/124