Improve readme to ease in to the topic

[bdestombe]

Is your feature request related to a problem? Please describe. You'd need to be well informed to understand the listed advanced capabilities. It would be nice to first state that

• A calibration is needed to get from Stokes to temperatures.
• The build-in calibration has its limitations.
• The build-in post-processing, such as file processing and plotting the results, are very limited.

Before listing all those advanced capabilities.