doc: Link to gexp research paper.

* doc/guix.texi (G-Expressions): Add reference to the 2017 GPCE paper.

Change-Id: I47f9357ee2715640e8ceb1f5d286d98bf6c55abc
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Merges: #7288

doc/guix.texi

@@ -12528,6 +12528,13 @@ package available on the build side, here's how you would do it:
                         @dots{})))
 @end lisp
 
+@quotation Going Further
+To learn more about the design of g-expressions, read
+@c https://dx.doi.org/10.1145/3136040.3136045 goes to ACM
+@uref{https://arxiv.org/abs/1709.00833, @i{Code Staging in
+GNU@tie{}Guix}} (2017).
+@end quotation
+
 The syntactic form to construct gexps is summarized below.
 
 @defmac #~@var{exp}