[doc] Removes mention of Nix in the README.md file.

That is too much of a development workflow and shouldn't appear
on the landing page.
11 jobs for readme-fix in 4 minutes and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #19569
flake8

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  Build
passed #19573
knl
argotk

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integration
containers

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passed #19571
integration
libnrm

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passed #19570
integration
nrm

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  Test
passed #19578
rapl
py.test

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passed #19574
integration
testHello

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passed #19575
integration
testListen

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passed #19576
performance integration
testPerfwrapper

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passed #19577
knl
testPower

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  Quality
passed #19579
integration
repoquality

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