githooks documentation: add a note about the +x mode

In a freshly initialized repo it is only necessary to rename the .sample
hooks, but when using older repos (initialized with older git init)
enabled the +x mode is still necessary - docuement this.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -20,6 +20,10 @@ directory to trigger action at certain points.  When
 all disabled.  To enable a hook, rename it by removing its `.sample`
 suffix.
 
+NOTE: It is also a requirement for a given hook to be executable.
+However - in a freshly initialized repository - the `.sample` files are
+executable by default.
+
 This document describes the currently defined hooks.
 
 applypatch-msg