Improve the documentation of the IDE-SECIR model

[lenaploetzke]

Motivation / Current Behaviour

Currently, the documentation in the README of the IDE-SECIR model is not very detailed.

Enhancement description

The documentation should be extended, especially by

• a section on initialization
• a diagram of the transitions (see also e.g. the ODE-SECIR model)
• a table showing the parameters used

Checklist

• [X] Attached labels, especially loc:: or model:: labels.
• [X] Linked to project