docs(errorcode.md): add the EcodeRootROnly error

Documentation/errorcode.md

@@ -21,6 +21,7 @@ Error code corresponding strerror
         EcodeNotDir         = 104
         EcodeNodeExist      = 105
         EcodeKeyIsPreserved = 106
+        EcodeRootROnly      = 107
 
         EcodeValueRequired     = 200
         EcodePrevValueRequired = 201
@@ -42,6 +43,7 @@ Error code corresponding strerror
     errors[104] = "Not A Directory"
     errors[105] = "Already exists" // create
     errors[106] = "The prefix of given key is a keyword in etcd"
+    errors[107] = "Root is read only"
 
     // Post form related errors
     errors[200] = "Value is Required in POST form"