Update comments in sd trace exporter readme.

[bogdandrutu]

No description provided.

[coveralls]

Coverage remained the same at 81.241% when pulling e302b4b on bogdandrutu:update-comments into 817f27b on census-instrumentation:master.

[codecov-io]

Codecov Report

Merging #815 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master    #815      +/-   ##
===========================================
+ Coverage     79.28%   79.3%   +0.02%     
  Complexity      899     899              
===========================================
  Files           156     156              
  Lines          3982    3982              
  Branches        372     372              
===========================================
+ Hits           3157    3158       +1     
  Misses          747     747              
+ Partials         78      77       -1

Impacted Files	Coverage Δ	Complexity Δ
...census/implcore/trace/export/SpanExporterImpl.java	98.33% <0%> (+1.66%)	7% <0%> (ø)	⬇️

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