Add changelog

Author: robx

changelog.d/pr-7921

@@ -0,0 +1,11 @@
+synopsis: Terminate subprocesses when killed
+packages: Cabal
+prs: #7921
+issues: #7914
+
+description: {
+
+- cabal (and 'cabal run' in particular) no longer leaves children running
+  when it is killed (unix)
+
+}