add help tip about nftables and ip tables

Author: jdv

crowdsec-docs/unversioned/bouncers/firewall.mdx

@@ -41,7 +41,14 @@ Supported firewalls:
 
 ## Installation
 
-Packages for crowdsec-firewall-bouncer [are available on our repositories](/docs/next/getting_started/install_crowdsec#install-our-repositories). You need to pick the package according to your firewall system :
+Packages for crowdsec-firewall-bouncer [are available on our repositories](/docs/next/getting_started/install_crowdsec#install-our-repositories). You need to pick the package according to your firewall system :  
+(info) *To know if your system is using iptables or nftables, you can run the following command:*
+
+```bash
+iptables -V
+```
+
+If you see the mention of 'nt_tables' in the output, you are using nftables.
 
 ### IPTables