Add deprecation notice to README

[lmuhlha]

No description provided.

[codecov]

Codecov Report

Merging #767 (9f003b8) into master (c6b5a6f) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #767      +/-   ##
============================================
- Coverage     55.21%   55.20%   -0.01%     
+ Complexity     3176     3175       -1     
============================================
  Files           751      751              
  Lines         20544    20544              
  Branches       1343     1343              
============================================
- Hits          11343    11341       -2     
- Misses         8715     8716       +1     
- Partials        486      487       +1     

Impacted Files	Coverage Δ	Complexity Δ
...com/spotify/heroic/aggregation/simple/MaxBucket.kt	44.44% <0.00%> (-11.12%)	4.00% <0.00%> (-1.00%)

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