Fix documentation on how to install optional features (issue #645)

[ods]

Changes

Fixes #645

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• 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.

[codecov]

Codecov Report

Merging #822 (e2f7a4f) into master (ccbcc25) will decrease coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #822      +/-   ##
==========================================
- Coverage   97.49%   96.96%   -0.54%     
==========================================
  Files          30       30              
  Lines        5434     5434              
==========================================
- Hits         5298     5269      -29     
- Misses        136      165      +29     

Flag	Coverage Δ
cext	87.74% <ø> (-0.58%)	⬇️
integration	96.92% <ø> (-0.54%)	⬇️
purepy	96.50% <ø> (-0.54%)	⬇️
unit	49.48% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiokafka/producer/sender.py	96.28% <0.00%> (-2.41%)	⬇️
aiokafka/client.py	96.64% <0.00%> (-1.53%)	⬇️
aiokafka/consumer/consumer.py	96.71% <0.00%> (-1.20%)	⬇️
aiokafka/producer/producer.py	97.56% <0.00%> (-0.98%)	⬇️
aiokafka/admin.py	87.38% <0.00%> (-0.94%)	⬇️
aiokafka/conn.py	93.23% <0.00%> (-0.44%)	⬇️
aiokafka/producer/message_accumulator.py	98.54% <0.00%> (-0.37%)	⬇️
aiokafka/consumer/fetcher.py	97.00% <0.00%> (-0.16%)	⬇️
aiokafka/consumer/group_coordinator.py	98.89% <0.00%> (-0.14%)	⬇️

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