CHANGELOG: give a hint about how to get type defs

[glasser]

The naming convention for DefinitelyTyped packages for namespaced packages is a bit obscure, so being explicit is nice (even if tsc does give the same hint).

[codecov]

Codecov Report

Merging #405 (eeccba9) into master (93f3f62) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #405   +/-   ##
=======================================
  Coverage   93.87%   93.87%           
=======================================
  Files           1        1           
  Lines         588      588           
=======================================
  Hits          552      552           
  Misses         36       36           

Flag	Coverage Δ
unit	93.87% <ø> (ø)

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