doc: add homebrew install

[chenrui333]

relates to #914

$ brew install crane
Updating Homebrew...
==> Downloading https://homebrew.bintray.com/bottles/crane-0.4.0.mojave.bottle.tar.gz
==> Downloading from https://d29vzk4ow07wi7.cloudfront.net/cc13961cd45cac3656448c7a3598a9805911ee6e76f5552aa26b3a14c55f57ba?response
######################################################################## 100.0%
==> Pouring crane-0.4.0.mojave.bottle.tar.gz
🍺  /usr/local/Cellar/crane/0.4.0: 5 files, 8.8MB

[codecov-io]

Codecov Report

Merging #919 (c78d560) into master (213dd48) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #919   +/-   ##
=======================================
  Coverage   72.77%   72.77%           
=======================================
  Files         108      108           
  Lines        4713     4713           
=======================================
  Hits         3430     3430           
  Misses        773      773           
  Partials      510      510           

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