Document broken LexRank pip implementation (#3567)

* Document broken LexRank pip implementation

Added a note about the broken pip implementation of LexRank and provided a link to a working implementation.

* Move comment as the linter complains, use link with `master`

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Co-authored-by: Tom Aarsen <[email protected]>

Author: stevenae

examples/sentence_transformer/applications/text-summarization/text-summarization.py

@@ -18,6 +18,9 @@
 Note: Requires NLTK: `pip install nltk`
 """
 
+# NB
+# The pip implementation of LexRank is broken.
+# A working implementation can be found at https://github.com/huggingface/sentence-transformers/blob/master/examples/sentence_transformer/applications/text-summarization/LexRank.py.
 import nltk
 import numpy as np
 from LexRank import degree_centrality_scores