docs: add note that the client is not compatible with the old-one

[bednar]

Closes #226

Proposed Changes

Added note that the client is not compatible with old-one.

Checklist

• Rebased/mergeable
• pytest tests completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov]

Codecov Report

Merging #227 (809c99e) into master (4015c90) will increase coverage by 1.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage   89.96%   90.99%   +1.02%     
==========================================
  Files          26       26              
  Lines        2003     2020      +17     
==========================================
+ Hits         1802     1838      +36     
+ Misses        201      182      -19     

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