Add: Survey on Text Augmentation

[amitness]

Hi @makcedward,

I recently wrote a survey article on data augmentation techniques for NLP. I have added the link to the "Extension Reading" section. Please review and merge it.

Article: https://amitness.com/2020/05/data-augmentation-for-nlp/

[codecov-commenter]

Codecov Report

Merging #130 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
- Coverage   50.05%   49.96%   -0.09%     
==========================================
  Files         129      129              
  Lines        4933     4933              
==========================================
- Hits         2469     2465       -4     
- Misses       2464     2468       +4     

Impacted Files	Coverage Δ
nlpaug/augmenter/word/word_augmenter.py	83.13% <0.00%> (-3.62%)	⬇️
nlpaug/augmenter/word/random.py	86.02% <0.00%> (-1.08%)	⬇️
nlpaug/augmenter/char/random.py	91.66% <0.00%> (ø)

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