added introduction to Locust4j

[myzhan]

Locust4j is a load generator for locust, written in Java.

Now, we can write test scenarios in Java.

[codecov]

Codecov Report

Merging #698 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #698      +/-   ##
==========================================
+ Coverage   66.09%   66.16%   +0.07%     
==========================================
  Files          16       16              
  Lines        1407     1407              
  Branches      215      215              
==========================================
+ Hits          930      931       +1     
  Misses        429      429              
+ Partials       48       47       -1

Impacted Files	Coverage Δ
locust/clients.py	89.88% <0%> (+1.12%)	⬆️

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[aldenpeterson-wf]

lgtm