update install guide to mention delay in Brew

[kerrywei]

Description

Call out the fact that release to Brew would take some time, and direct users to follow "from a pre-built binary" section in case they need the new version immediately

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Merging #428 (4cf5e80) into master (ee45c90) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   55.50%   55.50%           
=======================================
  Files          36       36           
  Lines        1906     1906           
=======================================
  Hits         1058     1058           
  Misses        711      711           
  Partials      137      137           

Flag	Coverage Δ
unittests	55.50% <ø> (ø)

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