Improve Readme description

[juliangruendner]

Update the readme with addtional information. The new readme should include:

1. A description of the fhir-data-evaluator including what it is used for and how it works. - Extending the Goal section and renaming it to overview.
2. An update on the input and output of the fhir data evaluator, including the patient count.
3. Examples for different measure inputs and excerpts of expected outputs as well as information on how to interpret these. - extending the Documentation already provided and summarizing the purpose of the fhir data evaluator