add: description how to install krew on fish

[nammn]

Adds a description section to install krew with fish.
This should close #179

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[codecov-io]

Codecov Report

Merging #187 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   50.91%   50.91%           
=======================================
  Files          13       13           
  Lines         711      711           
=======================================
  Hits          362      362           
  Misses        297      297           
  Partials       52       52

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[ahmetb]

What if mktemp returns a path with space in it?
Shouldnt you quote this to prevent splitting?

[nammn]

Hmm, good question. Actually, I am not that deep into fish, besides using it.
But yes, just to make sure, I'm gonna add this

[nammn]

Just saw this while reading through fish.
fish-shell/fish-shell#159 (comment).
It seems that subcommands in fish prevent space splitting.

[ahmetb]

/lgtm
/approve

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This pull-request has been approved by: ahmetb, nammn

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