Add README note to the debugging client

Signed-off-by: Anatole Denis <anatole@unverle.fr>

cmds/client/README.md

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+# DHCPv6 debug client
+
+This is a simple dhcpv6 client for use as a debugging tool with coredhcp
+
+***This is not a general-purpose DHCP client. This is only a testing/debugging tool for developing CoreDHCP***