docs: Add example for configuring a Linux Bridge using network_state variable

Signed-off-by: Wen Liang <wenliang@redhat.com>

README.md

@@ -1330,6 +1330,50 @@ network_state:
         dhcp: false
 ```
 
+Configuring the Linux bridge with custom multicast, stp options, along with a port
+with specific stp settings:
+
+```yaml
+network_state:
+  interfaces:
+    - name: br0
+      type: linux-bridge
+      state: up
+      bridge:
+        options:
+          gc-timer: 29657
+          group-addr: 01:80:C2:00:00:00
+          group-forward-mask: 0
+          group-fwd-mask: 0
+          hash-max: 4096
+          hello-timer: 0
+          mac-ageing-time: 300
+          multicast-last-member-count: 2
+          multicast-last-member-interval: 100
+          multicast-membership-interval: 26000
+          multicast-querier: false
+          multicast-querier-interval: 25500
+          multicast-query-interval: 12500
+          multicast-query-response-interval: 1000
+          multicast-query-use-ifaddr: false
+          multicast-router: auto
+          multicast-snooping: true
+          multicast-startup-query-count: 2
+          multicast-startup-query-interval: 3125
+          stp:
+            enabled: false
+            forward-delay: 15
+            hello-time: 2
+            max-age: 20
+            priority: 32768
+          vlan-protocol: 802.1q
+        port:
+          - name: eth1
+            stp-hairpin-mode: false
+            stp-path-cost: 100
+            stp-priority: 32
+```
+
 Configuring the route:
 
 ```yaml