To add information to Marcel's answer, the process is as follows:
1) Check if espaper.txt is available on the file system. If it is, load configuration values from there. If it is not, start configuration portal (access point mode, webserver) and display this on screen
2) Now the user has to connect by WiFi to the ESPaper access point and open the browser at the given (IP) address
3) The user enters the credentials for WiFi and wunderground and saves the configuration. Now the espaper.txt file exists on the drive
-> Restart the device and the code will load the settings from espaper.txt