Add recommendation to install hiredis

[Andrew-Chen-Wang]

What do these changes do?

Add docs about installing hiredis too since it is now an optional dependency.

Are there changes in behavior for the user?

Yes, installing hiredis is a completely different parser.

Related issue number

#1137 points out how hiredis resolved for issue.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES/ folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example:
    Fix issue with non-ascii contents in doctest text files.

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View rendered README.md

[codecov]

Codecov Report

Merging #1138 (4eebd3d) into master (4fbee2e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1138   +/-   ##
=======================================
  Coverage   89.23%   89.23%           
=======================================
  Files          21       21           
  Lines        6830     6830           
  Branches      654      654           
=======================================
  Hits         6095     6095           
  Misses        567      567           
  Partials      168      168           

Flag	Coverage Δ
unit	89.20% <ø> (ø)

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[Andrew-Chen-Wang]

Fixed in #1247