add app note: Communication between two SDR boards under AP and client mode

doc/app_notes/ap-client-two-sdr.md

@@ -1,2 +1,45 @@
-- To be done.
-- Tobe done
+- Power on two SDR boards. Call one board "AP board" and the other "client board". On each board, the TX and RX antenna should vertical/orthogonal to each other as much as possible to gain a good TX/RX isolation.
+- Connect a computer to the AP board via Ethernet cable. The computer should have static IP 192.168.10.1. Open a terminal on the computer, and then in the terminal:
+  ```
+  ssh root@192.168.10.122
+  (password: openwifi)
+  cd openwifi
+  ./fosdem.sh
+  (It will create a WiFi AP by hostapd program with config file: hostapd-openwifi.conf)
+  (Wait for the script completed)
+  cat /proc/interrupts
+  (Execute the "cat ..." command for several times)
+  (You should see the number of "sdr,tx_itrpt1" grows, because it sends the "openwifi" beacon periodically)
+  ```
+- Connect another computer to the client board via Ethernet cable. The computer should have static IP 192.168.10.1. Open a terminal on the computer, and then in the terminal:
+  ```
+  ssh root@192.168.10.122
+  (password: openwifi)
+  service network-manager stop
+  cd openwifi
+  ./wgd.sh
+  (Wait for the script completed)
+  ifconfig sdr0 up
+  iwlist sdr0 scan
+  (The "openwifi" AP should be listed in the scanning results)
+  iwconfig sdr0 essid openwifi
+  ```
+- Now the client is trying to associate with the AP. The AP board terminal should print like:
+  ```
+  ...
+  sdr0: AP-STA-CONNECTED 66:55:44:33:22:58
+  sdr0: STA 66:55:44:33:22:58 RADIUS: starting accounting session 1FF1C1B4-00000001
+  ```
+  If not, please adjust antenna/distance and re-run the commands on the client side.
+
+- After association is done, in the terminal of client:
+  ```
+  dhclient sdr0
+  (Wait for it completed)
+  ifconfig sdr0
+  (Now you should see the IP address like 192.168.13.x allocated by AP)
+  ./set_csma_normal.sh
+  ping 192.168.13.1
+  (Ping the AP)
+  ```
+  Now the communication link should be already setup between the AP and the client.