fixes #3206 remove references to waffle.io

[ssikdar1]

A PR to fix issue #3206 and remove links to waffle.io :

Files changed based on grep:

$ grep -rnw . -e 'waffle'
./README.md:250:via [Waffle.io](https://waffle.io/typelevel/cats).
./CONTRIBUTING.md:265:via [Waffle.io](https://waffle.io/typelevel/cats).

[kailuowang]

Thanks!

[codecov-io]

Codecov Report

Merging #3229 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3229   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         376      376           
  Lines        7412     7412           
  Branches      200      192    -8     
=======================================
  Hits         6897     6897           
  Misses        515      515

Flag	Coverage Δ
#scala_version_212	93.38% <ø> (+0.01%)	⬆️
#scala_version_213	92.81% <ø> (-0.02%)	⬇️

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