CodingGuidelines: Add a note to avoid assignments inside if()

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

Documentation/CodingGuidelines

@@ -89,6 +89,8 @@ For C programs:
    of "else if" statements, it can make sense to add braces to
    single line blocks.
 
+ - We try to avoid assignments inside if().
+
  - Try to make your code understandable.  You may put comments
    in, but comments invariably tend to stale out when the code
    they were describing changes.  Often splitting a function