Added Sensor Modes

Update README.md

typo: fields of downlink packets are in hexadecimal format.

Added functionality to change Duty Cycle via downlink

This release is the initial release for processing the downlinks. The function process_received_downlink reads the downlink payload and updates the TX_INTERVAL variable with the contents of payload. The downlinks are in hexadecimal format in the TTN console.

note: This is a basic release of processing downlinks and if larger numbers are sent via downlink, the MCU sleeps for this amount of time and there is a danger of unwanted wait times or even a crash. Further releases solves this problem along with other added functionalities.