Add definition of point-wise to README

[AaronDelaplane]

First, thank you for the wonderful libraries and documentation.

When reading the documentation I did not know what "point-wise" meant. A quick Google search produced mathematical explanations that were a bit over my head. I posted a question on the chat channel and @SystemFw provided a clear definition that I've included in this PR.

Perhaps others will not know what "point-wise" means and this addition will help them proceed with a little more understanding as they are working through the documentation.

It is kind of a long addition. I certainly understand if you'd like to keep the docs lean and pass on this PR.

Thanks for all the hard work.

[easel]

LGTM. The travis failure looks spurious to me.

[ceedubs]

Thanks, @AaronDelaplane! Great change. I've restarted the build that seemed to have a spurious failure.

[codecov-io]

Codecov Report

Merging #2512 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2512   +/-   ##
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  Coverage   95.38%   95.38%           
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  Files         357      357           
  Lines        6528     6528           
  Branches      281      281           
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  Hits         6227     6227           
  Misses        301      301

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