Improved SMS transmission documentation

[caronc]

Description:

Related issue (if applicable): #1247

This simple PR just improves the documentation associated with SMS messges on the README as per request.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

Testing

n/a

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.36%. Comparing base (0553bf7) to head (9de5fc5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1291   +/-   ##
=======================================
  Coverage   99.36%   99.36%           
=======================================
  Files         159      159           
  Lines       20724    20724           
  Branches     3725     3725           
=======================================
  Hits        20593    20593           
  Misses        121      121           
  Partials       10       10           

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