Update 2.16 release notes to mention new requirement for commit-msg hook

Since change Ice8ffd5c7, which added the --where option on the
git-trailers command, the miniumum version of git required is
2.15. Update the release notes to mention this.

Bug: Issue 12546
Change-Id: I1f691b05976aeecf897dcf878567c1acf2bdc93b
1 file changed