STM32WL LORA : add readme

Author: jeromecoutant

targets/TARGET_STM/README.md

@@ -75,6 +75,7 @@ This table summarizes the STM32Cube versions currently used in Mbed OS master br
 | L4          |    1.16.0    | https://github.com/STMicroelectronics/STM32CubeL4 |
 | L5          |    1.3.0     | https://github.com/STMicroelectronics/STM32CubeL5 |
 | WB          |    1.8.0     | https://github.com/STMicroelectronics/STM32CubeWB |
+| WL          |    1.0.0     | https://github.com/STMicroelectronics/STM32CubeWL |
 
 In Mbed OS repository, we try to minimize the difference between "official" and copied files.
 
@@ -103,6 +104,10 @@ Tool is not used in Mbed OS, but it can be useful for you.
 
 [STM32WB README](TARGET_STM32WB/README.md)
 
+### STM32WL
+
+[STM32WL README](TARGET_STM32WL/README.md)
+
 ### STM32H7
 
 [STM32H7 README](TARGET_STM32H7/README.md)

targets/TARGET_STM/TARGET_STM32WL/README.md

@@ -0,0 +1,66 @@
+# STM32WL family
+
+[st.com STM32WL family page](https://www.st.com/en/microcontrollers-microprocessors/stm32wl-series.html)
+
+This ST MCU family is dual-core : based on an Arm Cortex-M4 core and an Arm Cortex-M0+ core.
+
+Only M4 core is currently used by mbed-os applications.
+
+# Supported boards
+
+## NUCLEO_WL55JC
+
+[st.com STM32WL5x MCU page](https://www.st.com/en/microcontrollers-microprocessors/stm32wl5x.html)
+
+[st.com NUCLEO page](https://www.st.com/en/evaluation-tools/nucleo-wl55jc.html)
+
+[mbed.com NUCLEO page](https://os.mbed.com/platforms/ST-Nucleo-WL55JC/)
+
+- Total FLASH is 256KB
+- RAM: 64 KB
+
+# LoRa
+
+## MBED-OS support
+
+Lora radio is enabled in default STM32WL configuration
+
+## baremetal support
+
+Baremetal is supported.
+
+mbed_app.json:
+```
+{
+    "requires": ["bare-metal", "lora", "stm32wl-lora-driver"]
+}
+```
+
+
+## GPIO debug pins
+
+2 pins can be configured to check RX and TX activity.
+
+Example of mbed_app.json:
+```
+{
+    "target_overrides": {
+        "NUCLEO_WL55JC": {
+            "stm32wl-lora-driver.debug_rx": "LED1",
+            "stm32wl-lora-driver.debug_tx": "LED2"
+        }
+    }
+}
+```
+
+## mbed-os test
+
+| target              | platform_name | test suite                                         | result | elapsed_time (sec) | copy_method |
+|---------------------|---------------|----------------------------------------------------|--------|--------------------|-------------|
+| NUCLEO_WL55JC-ARMC6 | NUCLEO_WL55JC | connectivity-lorawan-tests-tests-lorawan-loraradio | OK     | 44.8               | default     |
+
+
+
+## Application example
+
+https://github.com/ARMmbed/mbed-os-example-lorawan/