Clarify cmdstan installation issue

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

This closes #479

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-commenter]

Codecov Report

Merging #480 (8835b05) into develop (da8a86c) will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #480      +/-   ##
===========================================
+ Coverage    77.93%   77.96%   +0.03%     
===========================================
  Files           30       30              
  Lines         8991     8991              
===========================================
+ Hits          7007     7010       +3     
+ Misses        1984     1981       -3     

Impacted Files	Coverage Δ
cmdstanpy/cmdstanpy/utils.py	70.48% <0.00%> (ø)
a/cmdstanpy/cmdstanpy/cmdstanpy/utils.py	74.35% <0.00%> (ø)
...runner/work/cmdstanpy/cmdstanpy/cmdstanpy/utils.py	71.06% <0.00%> (ø)
cmdstanpy/cmdstanpy/model.py	84.83% <0.00%> (+0.77%)	⬆️

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[mitzimorris]

LGTM!