Add initial consumer documentation

[vlovgr]

No description provided.

[codecov]

Codecov Report

Merging #138 into 0.20.x will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           0.20.x    #138      +/-   ##
=========================================
+ Coverage   93.04%   93.1%   +0.05%     
=========================================
  Files          48      50       +2     
  Lines        1194    1204      +10     
  Branches       84      87       +3     
=========================================
+ Hits         1111    1121      +10     
  Misses         83      83

Impacted Files	Coverage Δ
src/main/scala/fs2/kafka/Headers.scala	100% <100%> (ø)	⬆️
src/main/scala/fs2/kafka/Deserializer.scala	100% <100%> (ø)	⬆️
src/main/scala/fs2/kafka/Serializer.scala	100% <100%> (ø)	⬆️
...ain/scala/fs2/kafka/UnexpectedTopicException.scala	100% <100%> (ø)
.../main/scala/fs2/kafka/SerializationException.scala	100% <100%> (ø)

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