add new HACKING!

warn about Client.transient_for

diff --git a/HACKING b/HACKING
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+Woop, a HACKING document for openbox at last!
+
+Beware the Client.transient_for. It can be set to a !NULL value of TRAN_GROUP,
+which is not a valid pointer. You must ALWAYS check for TRAN_GROUP before
+following transient_for. When TRAN_GROUP is found, Client.group will always
+be !NULL. Some smart action should be taken using all members of the group in
+this case.