Document that gnu utils are required in mac

[naxhh]

Document that GNU binaries are required for the support scripts.

grep and sed are not part of the coreutils package so we need to install them as another package.

[naxhh]

/assign cornelius

as we talked about this in #krew slack

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In response to this:

/assign cornelius

as we talked about this in #krew slack

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[naxhh]

I guess not then. ping @corneliusweig

[ahmetb]

Thanks for taking a stab at this. We've discussed this before #364 but I think we can accept docs on this since more and more contributors will go through it.

[corneliusweig]

Thanks, I hope this will make it easier for OSX contributors to get set up.
Also related: #482

[naxhh]

Changes applied, hope it's good now.

[ahmetb]

thanks
/lgtm
/approve

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