Add information about potential firewall conflicts

Add a note about the NAT setup for firecracker guests to
connect with the host.

Author: schaefi

README.md

@@ -254,6 +254,12 @@ host from the TUN/TAP device to the outside world. There are many possible
 solutions available and the following describes a simple static IP and NAT
 based setup.
 
+   **_NOTE:_** Please check what tooling is managing the firewall on
+   your host and refer to its documentation on how to set up the
+   nat/postrouting rules. The information below assumes there is no
+   other firewall software active on your host and only serves the
+   purpose of an example setup.
+
 The proposed example works within the following requirements:
 
 * initrd_path must be set in the flake configuration