STMOD_CELLULAR: update readme

Author: jeromecoutant

components/cellular/COMPONENT_STMOD_CELLULAR/README.md

@@ -43,7 +43,6 @@ Here is the used mbed_app.json:
    },
     "target_overrides": {
         "DISCO_L496AG": {
-            "target.macros_add": ["CELLULAR_DEVICE=STModCellular"],
             "target.components_add": ["STMOD_CELLULAR"],
             "stmod_cellular.provide-default": "true"
         }
@@ -70,3 +69,33 @@ mbed compile -t GCC_ARM -m DISCO_L496AG --flash
 ````
 
 Then check the serial console (serial baudrate set to 115200 bit/s)
+
+## Board with STMOD+ Connector
+
+### DISCO_L496AG
+
+https://os.mbed.com/platforms/ST-Discovery-L496AG/
+
+Default HW configuration:
+- STMOD pins 1, 2, 3 and 4 are mapped to UART CTS/TX/RX/RTS
+
+BG96 expansion board is fully compatible.
+
+### DISCO_L4R9I
+
+https://os.mbed.com/platforms/DISCO-L4R9I/
+
+Default HW configuration:
+- STMOD pins 1 and 2 are mapped to SPI
+- STMOD pins 2 and 3 are mapped to UART TX/RX
+
+BG96 expansion board can be used but without flow control.
+
+### DISCO_H747I
+
+https://os.mbed.com/platforms/ST-Discovery-H747I/
+
+Default HW configuration:
+- STMOD pins 1, 2, 3 and 4 are mapped to SPI
+
+BG96 expansion board can not be used without solder bridges update