Document, improve schema modification workflow

[ivan-aksamentov]

🙋 Feature Request

🔦 Context

The data flowing through application can sometimes change, depending on new scientific ideas and the epidemiological situation. We value backward compatibility with the old file and URL formats, but changing the data schema and adapting I/O serialization code for the new format has proven to be a difficult task. It requires many steps. Many I/O-related pieces of the application depends on it being done correctly.

😯 Describe the feature

We need to thoroughly document the steps that need to be taken in order to migrate the application to the new version of the schema.

Some basics can be found here:
#757 (comment)

The best place is probably in the readme, in (or after) the "Schemas" section
https://github.com/neherlab/covid19_scenarios/blob/9469b995be/README.md#schemas

We might also consider improving the entire workflow and reorganizing the code to make migrations more convenient and less error prone.