Add AAA table description and example to Configuration.md (#2168)

**What I did**
    Add AAA table description and example to configuration.md 

**Why I did it**
   AAA table description and example are missing in configuration.md 

**How I verified it**
    No code change, not validate.

**Details if related**

doc/Configuration.md

@@ -1048,7 +1048,7 @@ This option sets the port which ntp will choose to send time update requests fro
 NOTE: If a Loopback interface is defined on the switch ntp will choose this by default, so this setting
 is **required** if the switch has a Loopback interface and the ntp peer does not have defined routes
 for that address.
- 
+
 ```
 {
 "NTP": {
@@ -1501,6 +1501,29 @@ The list of root ports, all possible breakout modes, and default breakout modes
 }
 ```
 
+### AAA
+
+The AAA table defined the method SONiC used for Authentication, Authorization and Accounting.
+The method could be:
+-   default
+-   local
+-   tacacs+
+-   radius
+
+```
+"AAA": {
+    "authentication": {
+       "login": "local"
+    },
+    "authorization": {
+       "login": "local"
+    },
+    "accounting": {
+       "login": "local"
+    }
+}
+```
+
 For Developers
 ==============