WIP/DOC: Practical guide to reading video files.

Here is a start on the documentation requested by @stefanv in #850.

It needs better links, both external and to other sections of the documentation within scikit-image. I made no attempt to get the sphinx right yet. This is just a draft.

Also, this is opinionated, but I attempt to show each project's strengths and shortcomings. As discussed in #850, I do not believe an optimal solution exists yet. Once this is ready to merge I will solicit the input of the creators of various projects I mention. I hope they will feel that I give each project a fair, if brief, treatment here.

Is this on the right track?

from each frame *in* a video