Add references to PRG analysis in README

Signed-off-by: Richard Decal <public@richarddecal.com>

README.rst

@@ -114,3 +114,11 @@ Note, to combine the coverage data from all the tox environments run:
       - ::
 
             PYTEST_ADDOPTS=--cov-append tox
+
+References
+===========
+* [Precision-Recall-Gain Curves: PR Analysis Done Right (2015) by Peter A. Flach and Meelis Kull](https://papers.nips.cc/paper/2015/file/33e8075e9970de0cfea955afd4644bb2-Paper.pdf)
+* [sklearn-compatible implementation](https://github.com/scikit-learn/scikit-learn/pull/24121) by [Bradley Fowler](https://github.com/bradleyfowler123)
+* [PRG curves](https://www.biostat.wisc.edu/~page/rocprg.pdf) by [David Page](https://www.biostat.wisc.edu/~page/)
+* [Blog post by Bradley Fowler](https://snorkel.ai/improving-upon-precision-recall-and-f1-with-gain-metrics/)
+* [Original implementation](https://github.com/meeliskull/prg) by [Meelis Kull](https://github.com/meeliskull)