Temperature conversion

Ideally unit conversion would be included in `THMThermodo`, but can be separated away into a more UI facing category. The goal is to offer the readings in a multitude of scales, which would allow displaying data based on either user preference or user locale, without forcing the 3rd party developer to include their own conversion logic (meaning adding units and/or changing internal logic to reach those units is transparent for the developer).

In addition, some tidying up in the `README.md` to fix a few oddities.