Modify README

Add descriptions of WPA configuration in IBSS mode.

Author: jychen0611

README.md

@@ -430,7 +430,9 @@ Now, assign an IP address to both interfaces using the following commands:
 $ sudo ip netns exec ns3 ip addr add 10.0.0.4/24 dev vw3
 $ sudo ip netns exec ns4 ip addr add 10.0.0.5/24 dev vw4
 ```
-#### Switch to IBSS mode
+There are two methods to configure an IBSS network: manual configuration or using WPA.
+#### Option1 : Manual configuration
+##### Switch to IBSS mode
 Switch device to IBSS mode using the following command : 
 
 ***iw dev [interface] set type ibss***
@@ -454,7 +456,7 @@ Interface vw3
 	wiphy 4
 	txpower 0.00 dBm
 ```
-#### Join IBSS network
+##### Join IBSS network
 ```shell
 $ sudo ip netns exec ns3 ip link set vw3 up
 $ sudo ip netns exec ns4 ip link set vw4 up
@@ -485,6 +487,44 @@ Interface vw3
 	wiphy 4
 	txpower 0.00 dBm
 ```
+#### Option2 : Using WPA
+```shell
+$ sudo ip netns exec ns3 ip link set vw3 up
+$ sudo ip netns exec ns4 ip link set vw4 up
+```
+Prepare the following script `wpa_supplicant_ibss.conf` (you can modify the script based on your needs):
+```shell
+network={
+    ssid="ibss1"
+    mode=1
+    frequency=2412
+    key_mgmt=WPA-PSK
+    proto=RSN
+    pairwise=CCMP
+    group=CCMP
+    psk="12345678"
+}
+```
+Using the command **wpa_supplicant**, configure `vw3` and `vw4` to join `ibss1`.
+```shell
+$ sudo ip netns exec ns3 wpa_supplicant -i vw3 -B -c scripts/wpa_supplicant_ibss.conf
+$ sudo ip netns exec ns4 wpa_supplicant -i vw4 -B -c scripts/wpa_supplicant_ibss.conf
+```
+Check the information of `vw3`.
+```shell
+$ sudo ip netns exec ns3 iw dev vw3 info
+```
+You should see output similar to the following:
+```
+Interface vw3
+	ifindex 6
+	wdev 0x400000001
+	addr 00:76:77:33:00:00
+	ssid ibss1
+	type IBSS
+	wiphy 4
+	txpower 0.00 dBm
+```
 #### Transmission/Receivement test
 To perform a ping test between two IBSS devices (`vw3` and `vw4`) in the same ibss cell (`ibss1`), use the following command:
 ```shell