Clarify Env module constants in README

[odow]

This question came up at INFORMS, and it has been asked multiple times on discourse.

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (5f32366) 90.88% compared to head (831a0ed) 90.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #521 +/- ## ======================================= Coverage 90.88% 90.88% ======================================= Files 5 5 Lines 2425 2425 ======================================= Hits 2204 2204 Misses 221 221 ```

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