[DOC] Add video

[jdkent]

Adds @adelavega's excellent video explaining pliers at the TOP, since people like watching things more than reading, and link corresponding tutorial/course in readme.

[adelavega]

As flattered as I am James, maybe the link to the tutorial is sufficient? Only because this is domain specific to fmri/neuroimaging and in theory pliers is a domain general tool.

[jdkent]

I think it's still good to have the video directly linked (I think it's a good sign that shows project vitality, investment, and people sometimes prefer to watch something versus read), but I conceded and put the video in Documentation with a disclaimer that pliers is a general-purpose tool.

If the video becomes too out of date we can remove it (or replace it?)

How does that sound?

[codecov-io]

Codecov Report

Merging #454 (42326c1) into master (0ed1f89) will decrease coverage by 0.31%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
- Coverage   75.78%   75.46%   -0.32%     
==========================================
  Files          65       65              
  Lines        3820     3779      -41     
==========================================
- Hits         2895     2852      -43     
- Misses        925      927       +2     

Impacted Files	Coverage Δ
pliers/converters/api/wit.py	45.65% <0.00%> (-2.27%)	:arrow_down:
pliers/diagnostics/diagnostics.py	88.29% <0.00%> (-2.13%)	:arrow_down:
pliers/support/decorators.py	80.00% <0.00%> (-1.82%)	:arrow_down:
pliers/transformers/api/google.py	40.00% <0.00%> (-1.18%)	:arrow_down:
pliers/transformers/api/microsoft.py	25.00% <0.00%> (-1.16%)	:arrow_down:
pliers/stimuli/api.py	37.73% <0.00%> (-1.16%)	:arrow_down:
pliers/converters/api/revai.py	25.75% <0.00%> (-1.11%)	:arrow_down:
pliers/converters/api/ibm.py	24.71% <0.00%> (-0.84%)	:arrow_down:
pliers/converters/base.py	90.00% <0.00%> (-0.63%)	:arrow_down:
pliers/extractors/api/clarifai.py	22.40% <0.00%> (-0.62%)	:arrow_down:
... and 12 more

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[jdkent]

for whatever reason, the doc build now requires pytest, I added pytest to the doc requirements and refactored setup.py so that the requirements in setup.py will be the same as the requirements in the files.

ready for review @adelavega