Add Rakuten to adopters list

[saucam]

Thank you for contributing to Cats!

This is a kind reminder to run sbt prePR and commit the changed files, if any, before submitting.

[codecov-io]

Codecov Report

Merging #2644 into master will increase coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2644      +/-   ##
==========================================
+ Coverage   94.86%   95.12%   +0.25%     
==========================================
  Files         363      363              
  Lines        6684     6704      +20     
  Branches      283      305      +22     
==========================================
+ Hits         6341     6377      +36     
+ Misses        343      327      -16

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/ContT.scala	100% <0%> (ø)	⬆️
laws/src/main/scala/cats/laws/MonadLaws.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/data/Ior.scala	98.54% <0%> (+0.01%)	⬆️
core/src/main/scala/cats/Eval.scala	98.82% <0%> (+0.04%)	⬆️
core/src/main/scala/cats/data/IorT.scala	97.79% <0%> (+0.04%)	⬆️
.../src/main/scala/cats/data/RepresentableStore.scala	87.5% <0%> (+4.16%)	⬆️
core/src/main/scala/cats/syntax/either.scala	99.26% <0%> (+12.5%)	⬆️
testkit/src/main/scala/cats/tests/CatsSuite.scala	70% <0%> (+36.66%)	⬆️

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