Working hook in platformio.ini

ESPFirewall/lib/Firewall/docs/firewall/firewall.tex

@@ -116,4 +116,11 @@ Looking at the printed protocols this means 6 == "TCP" and 17 == "UDP".
 
 After learning the protocols that need to be filtered and looked out for, a hook needs to be registered in order to filter the packets based on our rules.
 
+To register a hook \verb|LWIP_HOOK_IP4_INPUT| needs to be set as \verb|build_flag| in the \verb|platformio.ini| file to overwrite it in LwIP. This is an easy way of testing if it works as expected, but should be written in a function for any other use-case.
+
+\begin{verbatim}
+build_flags = 
+    '-DLWIP_HOOK_IP4_INPUT(pbuf, input_netif)=({ESP_LOGI("HOOK","TEST");0;})'
+\end{verbatim}
+
 \subsection{Benchmark}