Add nlp_primitives add-on to README, install instructions, and setup.py

[gsheni]

• Add nlp_primitives add-on to README, install instructions, and setup.py

[codecov]

Codecov Report

Merging #1743 (1e4549c) into main (629324d) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1743      +/-   ##
==========================================
+ Coverage   98.63%   98.69%   +0.06%     
==========================================
  Files         138      138              
  Lines       15368    15368              
==========================================
+ Hits        15158    15168      +10     
+ Misses        210      200      -10     

Impacted Files	Coverage Δ
featuretools/tests/conftest.py	100.00% <0.00%> (+0.40%)	⬆️
...utational_backend/test_calculate_feature_matrix.py	99.45% <0.00%> (+0.65%)	⬆️
...computational_backends/calculate_feature_matrix.py	99.42% <0.00%> (+0.85%)	⬆️

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[thehomebrewnerd]

Do we want to add the nlp primitives back to the API reference like we had before, or leave them out? I don't have a strong preference, but adding them back in would make nlp_primitives required for building the docs.

[gsheni]

@thehomebrewnerd I added back nlp_primitives to api_ref.
1e4549c