Add usage example.

[LLYX]

Add usage example for Token Bucket Rate Limiter.

[codecov-commenter]

Codecov Report

Merging #21 (246adc5) into master (2f060dc) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #21   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines           67        67           
  Branches        13        10    -3     
=========================================
  Hits            67        67           

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