docs: add example how to use Combine to prepare batches for write into InfluxDB

[bednar]

Closes #43

Proposed Changes

1. Added an example how to use Combine to prepare batches for write into InfluxDB

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• swift test completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.27%. Comparing base (d7e9b67) to head (6e90ffd).
⚠️ Report is 40 commits behind head on master.
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Additional details and impacted files

@@             Coverage Diff             @@
##           master      #44       +/-   ##
===========================================
+ Coverage   51.73%   87.27%   +35.53%     
===========================================
  Files         137       10      -127     
  Lines        6240      707     -5533     
===========================================
- Hits         3228      617     -2611     
+ Misses       3012       90     -2922     

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