Update Metrics Transform Processor README

[james-bebbington]

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[codecov]

Codecov Report

Merging #2142 (1a76fbb) into master (2b3acce) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2142      +/-   ##
==========================================
+ Coverage   90.40%   90.41%   +0.01%     
==========================================
  Files         397      397              
  Lines       19478    19478              
==========================================
+ Hits        17609    17612       +3     
+ Misses       1407     1406       -1     
+ Partials      462      460       -2     

Flag	Coverage Δ
integration	69.73% <ø> (ø)
unit	89.20% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
receiver/k8sclusterreceiver/watcher.go	95.29% <0.00%> (-2.36%)	⬇️
exporter/signalfxexporter/dimensions/requests.go	92.15% <0.00%> (+9.80%)	⬆️

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