Add 20-newsgroup lucene index guide

[x65han]

• Add a README on how to obtain Lucene index for 20-newsgroup dataset along with helper script

[codecov]

Codecov Report

Merging #1203 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1203      +/-   ##
============================================
+ Coverage     48.10%   48.16%   +0.05%     
- Complexity      727      731       +4     
============================================
  Files           147      147              
  Lines          8527     8527              
  Branches       1211     1211              
============================================
+ Hits           4102     4107       +5     
+ Misses         4085     4081       -4     
+ Partials        340      339       -1     

Impacted Files	Coverage Δ	Complexity Δ
...anserini/ltr/feature/base/PMIFeatureExtractor.java	86.53% <0.00%> (+1.92%)	13.00% <0.00%> (+1.00%)
...java/io/anserini/ltr/feature/CountBigramPairs.java	70.12% <0.00%> (+5.19%)	26.00% <0.00%> (+3.00%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5407b03...5f2d5fd. Read the comment docs.

[x65han]

Resolved all. Not sure if I got everything right. Let me know, please. Thanks!

[lintool]

Minor changes, otherwise looks good.