Makefile: discuss SHAttered in *_SHA{1,256} discussion

Let's mention the SHAttered attack and more generally why we use the
sha1collisiondetection backend by default, and note that for SHA-256
the user should feel free to pick any of the supported backends as far
as hashing security is concerned.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Makefile

@@ -481,6 +481,17 @@ include shared.mak
 #
 # === SHA-1 backend ===
 #
+# ==== Security ====
+#
+# Due to the SHAttered (https://shattered.io) attack vector on SHA-1
+# it's strongly recommended to use the sha1collisiondetection
+# counter-cryptanalysis library for SHA-1 hashing.
+#
+# If you know that you can trust the repository contents, or where
+# potential SHA-1 attacks are otherwise mitigated the other backends
+# listed in "SHA-1 implementations" are faster than
+# sha1collisiondetection.
+#
 # ==== Default SHA-1 backend ====
 #
 # If no *_SHA1 backend is picked, the first supported one listed in
@@ -525,6 +536,11 @@ include shared.mak
 #
 # === SHA-256 backend ===
 #
+# ==== Security ====
+#
+# Unlike SHA-1 the SHA-256 algorithm does not suffer from any known
+# vulnerabilities, so any implementation will do.
+#
 # ==== SHA-256 implementations ====
 #
 # Define OPENSSL_SHA256 to use the SHA-256 routines in OpenSSL.