docs: update tag.description.markdown documentation…

[Kafkalasch]

…with pointer to --markdownFiles flag

Describe the PR
Adds a brief line to the README to explain how to use the tag.description.markdown feature:
This should prevent questions like here: #1264
I also struggled a bit to find this out.

Relation issue
#1264

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6cdaaf5) 83.52% compared to head (46f74f0) 83.39%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1646      +/-   ##
==========================================
- Coverage   83.52%   83.39%   -0.14%     
==========================================
  Files          19       16       -3     
  Lines        3823     3781      -42     
==========================================
- Hits         3193     3153      -40     
+ Misses        534      532       -2     
  Partials       96       96              

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