Added cellular readme

Author: Ari Parkkila

features/cellular/README.md

@@ -0,0 +1,64 @@
+# Mbed Cellular Connectivity
+
+This is the Github repo for Mbed cellular connectivity:
+
+    easy_cellular/
+        EasyCellularConnection  Simplified cellular usage based on CellularBase.h
+        CellularConnectionUtil  An utility class for cellular connection
+
+    framework/
+        API         Application Programming Interface for cellular connectivity
+        AT          AT implementation based on 3GPP TS 27.007 specification
+        common      Common and utility sources
+        mux         MUX implementation based on 3GPP TS 27.010 specification
+        targets     Vendor specific cellular module adaptations
+
+## Known Limitations
+
+**Please note that as a first release framework is subject to change and may break code that uses it.**
+
+Only UDP is supported when using AT commands to control sockets in an IP stack built into the cellular modem. If TCP is required use PPP/LWIP stack.
+
+## Supported Modules
+
+Currently supported cellular modules can be found in the framework/targets/ folder, where also support for new cellular modules are added.
+
+## Cellular Configuration
+
+Cellular defaults can be changed in mbed_app.json configuration file:
+
+    "config": {
+        "cellular_plmn": {
+            "help": "PLMN selection, 0=auto",
+            "value": 0
+        },
+        "apn": {
+            "help": "Access point name, e.g. internet",
+            "value": "\"internet\""
+        },
+        "cellular_sim_pin": {
+            "help": "PIN code",
+            "value": "\"1234\""
+        }
+    }
+
+## Debug Traces
+
+Debug tracing level can be defined in mbed_app.json configuration file:
+
+    "target_overrides": {
+        "*": {
+            "target.features_add": ["COMMON_PAL"],
+            "mbed-trace.enable": true,
+            "platform.stdio-convert-newlines": true,
+            "platform.stdio-baud-rate": 115200,
+            "platform.default-serial-baud-rate": 115200
+        }
+    },
+    "config": {
+        "trace-level": {
+            "help": "Options are TRACE_LEVEL_ERROR,TRACE_LEVEL_WARN,TRACE_LEVEL_INFO,TRACE_LEVEL_DEBUG",
+            "macro_name": "MBED_TRACE_MAX_LEVEL",
+            "value": "TRACE_LEVEL_INFO"
+        }
+    }