Add missing files to src/backend/lib/README.

The README lists all the files available in the directory, along with short
descriptions of each, but a few newly added ones were missing. While we're
at it, reorder the list into alphabetical order.

Author: Takeshi Ideriha
Discussion: https://www.postgresql.org/message-id/4E72940DA2BF16479384A86D54D0988A56793487@G01JPEXMBKW04

src/backend/lib/README

@@ -3,16 +3,22 @@ in the backend:
 
 binaryheap.c - a binary heap
 
+bipartite_match.c - Hopcroft-Karp maximum cardinality algorithm for bipartite graphs
+
 bloomfilter.c - probabilistic, space-efficient set membership testing
 
+dshash.c - concurrent hash tables backed by dynamic shared memory areas
+
 hyperloglog.c - a streaming cardinality estimator
 
+ilist.c - single and double-linked lists
+
+knapsack.c - knapsack problem solver
+
 pairingheap.c - a pairing heap
 
 rbtree.c - a red-black tree
 
-ilist.c - single and double-linked lists.
-
 stringinfo.c - an extensible string type