sagemathgh-39779: Fix reference in chrompoly.pyx and add to master reference file
    
This PR fixes sagemath#39768
incorrect reference in the chrompoly.pyx file is fixed and the reference
has been updated to the correct citation and added to the master
reference file (index.rst).
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URL: sagemath#39779
Reported by: Krishna Dubey
Reviewer(s): David Coudert

Author: Release Manager

src/doc/en/reference/references/index.rst

@@ -5652,6 +5652,10 @@ REFERENCES:
               Congressus numerantium, 1994.
               Pages 97--110
 
+.. [Rea1968] Ronald C. Read,
+             An improved method for computing the chromatic polynomials of sparse graphs,
+             Research Report CORR 87-20, C & O Dept. Univ. of Waterloo, 1987.
+
 .. [Rea2004] Nathan Reading.
              *Cambrian Lattices*.
              :arxiv:`math/0402086v2`.

src/sage/graphs/chrompoly.pyx

@@ -10,8 +10,9 @@ AUTHORS:
 
 REFERENCE:
 
-    Ronald C Read, An improved method for computing the chromatic polynomials of
-    sparse graphs.
+    See [Rea1968]_ and the :wikipedia:`Chromatic_polynomial` for more details
+    on this notion in graphs.
+
 """
 
 # ****************************************************************************
@@ -41,7 +42,7 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
     Compute the chromatic polynomial of the graph G.
 
     The algorithm used is a recursive one, based on the following observations
-    of Read:
+    of Read [Rea1968]_:
 
         - The chromatic polynomial of a tree on n vertices is x(x-1)^(n-1).