Providing citation for DFS

[kveerama]

Providing citation for DFS in readme.

[codecov-io]

Codecov Report

Merging #243 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
- Coverage   93.65%   93.65%   -0.01%     
==========================================
  Files          71       71              
  Lines        7697     7690       -7     
==========================================
- Hits         7209     7202       -7     
  Misses        488      488

Impacted Files	Coverage Δ
featuretools/synthesis/deep_feature_synthesis.py	93.22% <0%> (-0.06%)	⬇️
...ols/tests/feature_function_tests/test_agg_feats.py	98.52% <0%> (-0.03%)	⬇️

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[kmax12]

Looks good. Merging