Document PicklingError for ensure_annotations in multiprocessing

[KeyWeeUsr]

Closes #29 with adding a warning to the documentation.

[codecov-io]

Codecov Report

Merging #32 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files           3        3           
  Lines         513      513           
=======================================
  Hits          463      463           
  Misses         50       50

Impacted Files	Coverage Δ
ensure/main.py	89.47% <ø> (ø)	⬆️

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[kislyuk]

Thanks!