Add Earnest as adopter

[BlackVegetable]

I am a recently hired engineer at Earnest. I noticed Earnest uses circe but isn't on your list. I was told to submit a PR and change that!

[codecov-io]

Codecov Report

Merging #807 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #807      +/-   ##
==========================================
+ Coverage   86.85%   86.89%   +0.04%     
==========================================
  Files          71       71              
  Lines        2290     2290              
  Branches      160      164       +4     
==========================================
+ Hits         1989     1990       +1     
+ Misses        301      300       -1

Impacted Files	Coverage Δ
...re/shared/src/main/scala/io/circe/JsonNumber.scala	96.03% <0%> (-1%)	⬇️
...rc/main/scala/io/circe/shapes/SizedInstances.scala	92.85% <0%> (+14.28%)	⬆️

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[travisbrown]

Great, thanks!