Adding hint to mbed_app.json if no SIM pin is necessary

Author: bridadan

README.md

@@ -30,7 +30,7 @@ See the file `mbed_app.json` in the root directory of your application. This fil
 
 ```json
         "sim-pin-code": {
-            "help": "SIM PIN code",
+            "help": "SIM PIN code, set to 0 if none",
             "value": "\"1234\""
         },
         "apn": {

mbed_app.json

@@ -2,7 +2,7 @@
     "config": {
         "sock-type": "TCP",
         "sim-pin-code": {
-            "help": "SIM PIN code",
+            "help": "SIM PIN code, set to 0 if none",
             "value": "\"1234\""
         },
         "apn": {