📚 add usage warning

[chfw]

With your PR, here is a check list:

• Has test cases written?
• Has all code lines tested?
• Has make format been run?
• Please update CHANGELOG.yml(not CHANGELOG.rst)
• Has fair amount of documentation if your change is complex
• Agree on NEW BSD License for your contribution

[codecov-io]

Codecov Report

Merging #44 (5013cca) into dev (ced153d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev      #44   +/-   ##
=======================================
  Coverage   98.40%   98.40%           
=======================================
  Files          13       13           
  Lines         690      690           
=======================================
  Hits          679      679           
  Misses         11       11           

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