doc: mention "clear" cli command in README

[Delapouite]

Hi

This PR helps discovering this clear command.

[codecov]

Codecov Report

Merging #40 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   50.39%   50.39%           
=======================================
  Files           9        9           
  Lines         764      764           
=======================================
  Hits          385      385           
  Misses        379      379           

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[kingosticks]

Thank you.