Add note on thread-safety to README

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (712e61d) 83.34% compared to head (5256b3f) 83.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #182 +/- ## ======================================= Coverage 83.34% 83.34% ======================================= Files 3 3 Lines 1633 1633 ======================================= Hits 1361 1361 Misses 272 272 ```

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[odow]

Merging since this is an improvement. I'll leave #181 open for now