added docs on withEnv & withGlobalEnv

[codezninja]

fixes #293

[codecov-commenter]

Codecov Report

Merging #295 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #295   +/-   ##
=======================================
  Coverage   76.04%   76.04%           
=======================================
  Files          38       38           
  Lines         981      981           
  Branches      233      233           
=======================================
  Hits          746      746           
  Misses        122      122           
  Partials      113      113           

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[kmanning]

Can you update the CHANGELOG to include your change?

[kmanning]

Otherwise, looks great, thanks!