Write a regions notebook in astropy-tutorials

As suggested by @astrofrog and me at https://github.com/astropy/regions/pull/87#issuecomment-264002104, we should write a "getting started" notebook for `regions`, and put it in `astropy-tutorials`.

I think a 5 to 10 min read would be appropriate, and start and end by pointing at the longer Sphinx docs.

I'm putting this under the 0.2 milestone, but this can happen after the release. It should happen before the 0.2 announcement to astropy-dev though, so that this can be mentioned.

Anyone interested to do this?

If no, I'll write it on Monday latest.

Review the longer Sphinx documentation for regions and the astropy-tutorials location mentioned in the issue. Create a 5–10 minute getting-started notebook that points to the longer documentation at the beginning and end; completion means the notebook is ready to be included before the announcement.

Written by the indexing model from the issue text.