Add information about dftd4 in spack

[awvwgk]

Closes #94

[codecov]

Codecov Report

Merging #116 (752052c) into main (00e2ddc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #116   +/-   ##
=======================================
  Coverage   59.16%   59.16%           
=======================================
  Files          40       40           
  Lines        2946     2946           
  Branches      995      995           
=======================================
  Hits         1743     1743           
  Misses        642      642           
  Partials      561      561           

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