Added readme file

Author: Wajahat Abbas

features/cellular/framework/targets/UBLOX/README.md

@@ -0,0 +1,27 @@
+## Configuring Ublox Cellular Modems for custom targets
+
+In order to use cellular APIs with custom targets, user needs to define ublox modem macros in `mbed_app.json` configuration file:
+```
+"target_overrides": {
+    "*": {
+        "mbed-trace.enable": true,
+        "platform.stdio-convert-newlines": true,
+        "platform.stdio-baud-rate": 115200,
+        "platform.default-serial-baud-rate": 115200
+    },
+    "K64F": {
+        "target.macros_add": ["UBX_MDM_SARA_R41XM", "UBX_MDM_SARA_R412M"]
+    }
+}
+```
+
+
+List of supported ublox modems and their respective macros are as follow:
+
+| Ublox mbed board  | Modem Name    | Family Macro         | Modem Macro          |
+| -----------       | :-----------: | :-------------:      | :-------------:      |
+| UBLOX_C027        | SARA-G350     | `UBX_MDM_SARA_G3XX`  | `UBX_MDM_SARA_G350`  |
+| UBLOX_C030_N211   | SARA-N211     | `UBX_MDM_SARA_N2XX`  | `UBX_MDM_SARA_N211`  |
+| UBLOX_C030_U201   | SARA-U201     | `UBX_MDM_SARA_U2XX`  | `UBX_MDM_SARA_U201`  |
+| UBLOX_C030_R410M  | SARA-R410M    | `UBX_MDM_SARA_R41XM` | `UBX_MDM_SARA_R410M` |
+| UBLOX_C030_R412M  | SARA-R412M    | `UBX_MDM_SARA_R41XM` | `UBX_MDM_SARA_R412M` |