adding asdf observation to readme file

[rafaelgallani]

Just a simple addition. 😸
I spent quite some time trying whether I had missed something in the installation, I think it might help others.

[codecov-io]

Codecov Report

Merging #13 (3970472) into master (011269a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   91.00%   91.00%           
=======================================
  Files           1        1           
  Lines         189      189           
=======================================
  Hits          172      172           
  Misses         17       17           

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[noctuid]

Thanks!