Add link to UnitfulEquivalences.jl to README

[sostock]

I just registered the UnitfulEquivalences package. This PR adds a link to it to the README.

[codecov-io]

Codecov Report

Merging #401 (c193b0d) into master (839e30e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #401   +/-   ##
=======================================
  Coverage   83.20%   83.20%           
=======================================
  Files          15       15           
  Lines        1280     1280           
=======================================
  Hits         1065     1065           
  Misses        215      215           

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