Updated README.md

[ja8zyjits]

Made the optional Environment variable declaration for installation more clear.

[codecov]

Codecov Report

Merging #1444 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1444      +/-   ##
==========================================
+ Coverage   91.32%   91.38%   +0.05%     
==========================================
  Files          17       17              
  Lines        1718     1718              
  Branches      322      322              
==========================================
+ Hits         1569     1570       +1     
  Misses        123      123              
+ Partials       26       25       -1

Impacted Files	Coverage Δ
sanic/router.py	95.89% <0%> (+0.45%)	⬆️

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