cabal-install changelog update for Backpack.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

cabal-install/changelog

@@ -24,6 +24,9 @@
 	'.../$pkgid.log' to '.../$compiler/$libname.log' (#3807).
 	* Added a new command, 'cabal reconfigure', which re-runs 'configure'
 	with the most recently used flags (#2214).
+	* Support for building Backpack packages.  See
+	https://github.com/ezyang/ghc-proposals/blob/backpack/proposals/0000-backpack.rst
+	for more details.
 
 1.24.0.0 Ryan Thomas <ryan@ryant.org> March 2016
 	* If there are multiple remote repos, 'cabal update' now updates