Add a .env_sample file and Update README #447

[trangttt]

#447

@mbernier edit: closes #447

[SendGridDX]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #449 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #449   +/-   ##
=======================================
  Coverage   82.38%   82.38%           
=======================================
  Files           9        9           
  Lines         999      999           
  Branches      160      160           
=======================================
  Hits          823      823           
  Misses         86       86           
  Partials       90       90

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[thinkingserious]

Hello @trangttt,

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