added information about stats/accounting and new configuration directives

README.md

 ## Example IPv4
 `iptables -A INPUT -p udp -m udp --dport 67 -j NFQUEUE --queue-num 67 --queue-bypass`
 
+`iptables -A OUTPUT -p udp -m udp --sport 67 -j NFQUEUE --queue-num 67 --queue-bypass` (optional for accounting)
+
 ## Example IPv6
 `ip6tables -A INPUT -p udp -m udp --dport 547 -j NFQUEUE --queue-num 67 --queue-bypass`
 
+`ip6tables -A OUTPUT -p udp -m udp --sport 547 -j NFQUEUE --queue-num 67 --queue-bypass` (optional for accounting)
+
 # Configuration
 The configuration file may be tuned, but the defaults should be fine.
 ```
 # actions.
 # Set to 0 for production.
 dryrun=0
+
+# stats file and interval (seconds)
+# the stats file will be overwritten every n seconds
+# as defined below. The counters are absolute AND
+# relative (both included).
+stats_file=/var/run/dhcp_protect_status.json
+stats_interval=10
 ```
 
 # Starting / Stopping
 
 # Logging / Accounting
 
+## Syslog
 The program will log every blacklisting action to syslog (also in dryrun mode).
 
 ```
 Oct 23 16:50:18 router dhcp_protect[9706]: 00000000021b: blacklisting started
 Oct 23 16:52:18 router dhcp_protect[9706]: 00000000021b: blacklisting ended
 ```
+
+## Status
+DHCP Protect will update its status in `stats_file` every `stats_interval` seconds. This is a JSON file containing the UNIX timestamp when the file was created/updated, absolute and relative (relative to late update) counters of all different DHCPv4/DHCPv6 messages types and the current blacklist entries.
+
+For example:
+
+```
+root@dhcp:~# cat /var/run/dhcp_protect_status.json | json_pp
+{
+   "dhcpv4" : {
+      "DISCOVER" : {
+         "abs" : 2,
+         "rel" : 0
+      },
+      "ACK" : {
+         "rel" : 0,
+         "abs" : 52
+      },
+      "REQUEST" : {
+         "rel" : 0,
+         "abs" : 52
+      },
+      "OFFER" : {
+         "rel" : 0,
+         "abs" : 1
+      }
+   },
+   "timestamp" : 1572204894,
+   "blacklist" : [],
+   "dhcpv6" : {
+      "RECONFIGURE-REQUEST" : {
+         "abs" : 61,
+         "rel" : 0
+      },
+      "SOLICIT" : {
+         "rel" : 0,
+         "abs" : 83
+      },
+      "RELEASE" : {
+         "rel" : 0,
+         "abs" : 6
+      },
+      "DECLINE" : {
+         "abs" : 61,
+         "rel" : 0
+      },
+      "RENEW" : {
+         "rel" : 0,
+         "abs" : 22
+      },
+      "REBIND" : {
+         "rel" : 0,
+         "abs" : 2
+      },
+      "RELAY-FORW" : {
+         "rel" : 0,
+         "abs" : 61
+      }
+   }
+}
+```
 # Copyright / License
 ```
    Copyright 2019 Pascal Gloor