v0.28.3: update changelog for security release

docs/changelog.md

+v0.28.3
+-------
+
+This is a security release fixing the following issues:
+
+* A carefully constructed commit object with a very large number
+  of parents may lead to potential out-of-bounds writes or
+  potential denial of service.
+
+* The ProgramData configuration file is always read for compatibility
+  with Git for Windows and Portable Git installations.  The ProgramData
+  location is not necessarily writable only by administrators, so we
+  now ensure that the configuration file is owned by the administrator
+  or the current user.
+
 v0.28.2
 -------