doc: add homebrew install

[chenrui333]

relates to #296

==> Downloading https://homebrew.bintray.com/bottles/ko-0.7.2.mojave.bottle.tar.gz
==> Downloading from https://d29vzk4ow07wi7.cloudfront.net/16cd2e751b78b91bd6e65b8b396afbfad8ce211942adf9eb4b602c25e8a5b9df?response
######################################################################## 100.0%
==> Pouring ko-0.7.2.mojave.bottle.tar.gz
🍺  /usr/local/Cellar/ko/0.7.2: 5 files, 35.4MB

[codecov-io]

Codecov Report

Merging #301 (7a06f19) into master (dfe3d51) will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   37.97%   38.10%   +0.13%     
==========================================
  Files          33       33              
  Lines        1501     1501              
==========================================
+ Hits          570      572       +2     
+ Misses        841      839       -2     
  Partials       90       90              

Impacted Files	Coverage Δ
pkg/publish/options.go	20.68% <0.00%> (-2.39%)	⬇️
pkg/commands/resolver.go	11.93% <0.00%> (+0.32%)	⬆️
pkg/publish/default.go	64.61% <0.00%> (+1.12%)	⬆️

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