RelNotes: mention "sha1dc: optionally use sha1collisiondetection as a submodule"

To note that merely cloning git.git without --recurse-submodules
doesn't get you a full copy of the code anymore. See
5f6482d642 ("RelNotes: mention "log: make --regexp-ignore-case work
with --perl-regexp"", 2017-07-20).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: avar

Documentation/RelNotes/2.14.0.txt

@@ -235,6 +235,11 @@ Performance, Internal Implementation, Development Support etc.
    behaviour of the comparison function can be specified at the time a
    hashmap is initialized.
 
+ * The "collision detecting" SHA-1 implementation shipped with 2.13 is
+   now integrated into git.git as a submodule (the first submodule to
+   ship with git.git). Clone git.git with --recurse-submodules to get
+   it. For now a non-submodule copy of the same code is also shipped
+   as part of the tree.
 
 Also contains various documentation updates and code clean-ups.