The subject of "configuration" has two aspects -- the various settings which are most commonly represented in a configuration file (most likely appsettings.json), and the setup of several library services which must be configured in code. This section is about the former. For the latter, please refer to the Library Services documentation.
The library defines a set of configuration classes designed to be populated by the Microsoft.Extensions.Configuration.* packages (aka MEC). The root of this hierarchy is EventStreamDotNetConfig and the only required group of values are in DatabaseConfig, namely, the database connection string, the event table name, and the snapshot table name. However, there are several other settings you can control.