Add kubectl plugin installation to Homebrew formula about kubectx

[ahmetb]

Honestly not a bad idea but it might confuse people a bit regarding where these plugins are from if they also have Krew installed on their system but these plugins aren't showing up on kubectl krew list. (For example, I use krew for other plugins but not for kubectx/kubens.)

[justenstall]

I can understand that. Users could still list these plugins with kubectl plugin list if they are installed by Homebrew, which I think is the most "correct" way to check the available plugins. There is precedent for distributing the plugin form through Homebrew, kubelogin does this (their implementation is different than the snippet I have in the original issue and possibly more correct).

[ahmetb]

Yeah makes sense. Go for it, I see no harm except when this shadows krew installation for the same plugin and vice versa, but it's not a big deal.

[justenstall]

This was implemented with Homebrew/homebrew-core#140831