Remove documentation in README about process.env types

[jcblw]

This just revises the README to not say that there will be different types on the process.env. Open to rewording this.

[codecov-io]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #10   +/-   ##
=======================================
  Coverage   97.77%   97.77%           
=======================================
  Files           1        1           
  Lines          45       45           
  Branches       16       16           
=======================================
  Hits           44       44           
  Misses          1        1

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