STMOD_CELLULAR: Read me update to describe test procedure

Author: jeromecoutant

components/cellular/COMPONENT_STMOD_CELLULAR/README.md

@@ -4,10 +4,69 @@ This library allows enabling the cellular module that is connected to the STMOD+
 
 ## Supported modems.
 
-Currently supported cellular modules are Quectel UG96 and BG96 expansion boards that are included in [p-l496g-cell01] (https://www.st.com/en/evaluation-tools/p-l496g-cell01.html) and [p-l496g-cell02.] (https://www.st.com/en/evaluation-tools/p-l496g-cell02.html) packs.
+Currently supported cellular modules are Quectel UG96 and BG96 expansion boards that are included in [p-l496g-cell01](https://www.st.com/en/evaluation-tools/p-l496g-cell01.html)
+and [p-l496g-cell02](https://www.st.com/en/evaluation-tools/p-l496g-cell02.html) packs.
 
 ## Specifications
 
-The STMOD+ Connector specification can be found [here] (https://www.st.com/content/ccc/resource/technical/document/technical_note/group0/04/7f/90/c1/ad/54/46/1f/DM00323609/files/DM00323609.pdf/jcr:content/translations/en.DM00323609.pdf).
+The STMOD+ Connector specification can be found [here](https://www.st.com/content/ccc/resource/technical/document/technical_note/group0/04/7f/90/c1/ad/54/46/1f/DM00323609/files/DM00323609.pdf/jcr:content/translations/en.DM00323609.pdf).
 
+## Cellular tests in mbed-os
 
+- features-cellular-tests-api-cellular_device
+- features-cellular-tests-api-cellular_information
+- features-cellular-tests-api-cellular_network
+- features-cellular-tests-api-cellular_sms
+- features-cellular-tests-socket-udp
+
+Here is the used mbed_app.json:
+
+````
+{
+    "config": {
+        "cellular-sim-pin" : {
+            "help": "PIN code",
+            "value": "\"1234\""
+        },
+        "apn": {
+            "help": "The APN string to use for this SIM/network, set to 0 if none",
+            "value": "\"APN\""
+        },
+        "username": {
+            "help": "The user name string to use for this APN, set to zero if none",
+            "value": 0
+        },
+        "password": {
+            "help": "The password string to use for this APN, set to 0 if none",
+            "value": 0
+        }
+   },
+    "target_overrides": {
+        "DISCO_L496AG": {
+            "target.macros_add": ["CELLULAR_DEVICE=STModCellular"],
+            "target.components_add": ["STMOD_CELLULAR"],
+            "stmod_cellular.provide-default": "true"
+        }
+    }
+}
+````
+
+
+````
+$ mbed test -t ARM -m DISCO_L496AG -v -n features-cellular-*
+````
+
+## Cellular mbed-os example
+
+https://github.com/ARMmbed/mbed-os-example-cellular
+
+````
+mbed import mbed-os-example-cellular
+cd mbed-os-example-cellular
+ 
+<edit mbed_app.json file as explained in the example: APN, PIN,... >
+ 
+mbed compile -t GCC_ARM -m DISCO_L496AG --flash
+````
+
+Then check the serial console (serial baudrate set to 115200 bit/s)