wsd, coding style: document double review for security

Document that authentication changes need double amount of care,
compared to other normal functional changes.

Signed-off-by: Miklos Vajna <vmiklos@collabora.com>
Change-Id: Ib1820581f7175957f08f5e080d542c7441a3ffa2

wsd/README

@@ -408,3 +408,11 @@ The style is roughly as follows, in rough order of importance:
 
   In other cases it makes the code more readable to still spell out the type
   explicitly.
+
+Security credential related changes
+-----------------------------------
+
+- Instead of the usual one, two reviews are needed.
+
+- Instead of just choosing the 'approve' option on GitHub, please add your
+  explicit sign-off to the commit message when you review.