Update README about librdkafka version dependency

[carsonip]

No description provided.

[codecov-io]

Codecov Report

Merging #927 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
+ Coverage   83.21%   83.39%   +0.18%     
==========================================
  Files          36       36              
  Lines        3806     3806              
  Branches      562      562              
==========================================
+ Hits         3167     3174       +7     
+ Misses        490      485       -5     
+ Partials      149      147       -2

Impacted Files	Coverage Δ
pykafka/balancedconsumer.py	89.47% <0%> (-0.3%)	⬇️
pykafka/simpleconsumer.py	85.04% <0%> (+0.44%)	⬆️
pykafka/broker.py	93.64% <0%> (+0.57%)	⬆️
pykafka/managedbalancedconsumer.py	84.76% <0%> (+0.66%)	⬆️
pykafka/cluster.py	72.16% <0%> (+0.73%)	⬆️
pykafka/handlers.py	93.38% <0%> (+0.73%)	⬆️
pykafka/connection.py	84.15% <0%> (+0.99%)	⬆️

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