Sensor integration guide

[jmadden173]

Describe the Enhancement
Create .md documentation for adding sensor support for new sensors on the platform. All currently supported sensor protocol should be referenced including analog measurements, sdi-12, and i2c. Each protocol should have common function calls for refernece (eg. SDI12_ReadMeasurement) so the sensor library can be easily implemented.

For now lets leave protobuf definitions out of the picture as our current interface is clunky requiring a completely new Measurement definition to be implemented along with corresponding encode/decode functions. We can consider alternative interfaces in the next major release.

Benefits
More documentation allowing non-lab collaborators to integrate new sensors with the system.

Possible Drawbacks
N/A

Additional Context
As part of #38, they'll be a library exposed for Arduino based i2c sensors to interface with the host device that can be documented in that issue.