mbed_app.json configured to operate on senet

To enable cayenne, edit mbed_app.json, assign pins for desired function

Example:

        "cayenne": {
            "help": "compile main_cayenne.cpp instead of main.cpp",
            "value": 1
        },
        "analog_in_pin": {
            "help": "PA_0, PA_4, PA_5",
            "value": "PA_0"
        },
        "analog_out_pin": {
            "help": "PA_0, PB_5, PB_10, PB_11",
            "value": "PB_11"
        },
        "_digital_out_pin": {
            "help": "",
            "value": ""
        }
Revision:
52:1bd775571014
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/CONTRIBUTING.md	Mon Apr 29 14:02:07 2019 +0100
@@ -0,0 +1,5 @@
+# Contributing to Mbed OS
+
+Mbed OS is an open-source, device software platform for the Internet of Things. Contributions are an important part of the platform, and our goal is to make it as simple as possible to become a contributor.
+
+To encourage productive collaboration, as well as robust, consistent and maintainable code, we have a set of guidelines for [contributing to Mbed OS](https://os.mbed.com/docs/mbed-os/latest/contributing/index.html).