docs: remove leftover reduce setting

[joscha]

Fixed some leftovers from #90. It's also quite confusing currently to have the docs for the new generate option, but it not being released yet. Any opts.generate is just silently ignored at the moment, I think either a release or backwards compatibility would be good.

[codecov-io]

Codecov Report

Merging #92 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #92   +/-   ##
======================================
  Coverage    98.8%   98.8%           
======================================
  Files           2       2           
  Lines          84      84           
======================================
  Hits           83      83           
  Misses          1       1

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[mastilver]

Thank you @joscha

I will be releasing v2 by the end of the week, I know I could have done a better job for crafting this release, but that's the way it is now ;)

See https://github.com/danethurber/webpack-manifest-plugin/blob/1.x/README.md for current docs