#include "FS.h"

void setup() {
  Serial.begin(115200);

  // always use this to "mount" the filesystem
  bool result = SPIFFS.begin();
  Serial.println("SPIFFS opened: " + result);

  // this opens the file "f.txt" in read-mode
  File f = SPIFFS.open("/f.txt", "r");
  
  if (!f) {
    Serial.println("File doesn't exist yet. Creating it");

    // open the file in write mode
    File f = SPIFFS.open("/f.txt", "w");
    if (!f) {
      Serial.println("file creation failed");
    }
    // now write two lines in key/value style with  end-of-line characters
    f.println("ssid=abc");
    f.println("password=123455secret");
  } else {
    // we could open the file
    while(f.available()) {
      //Lets read line by line from the file
      String line = f.readStringUntil('n');
      Serial.println(line);
    }

  }
  file.close();
}

void loop() {
  // nothing to do for now, this is just a simple test

}

You can also find the example here:
https://github.com/squix78/esp8266-projects/tree/master/arduino-ide/filesystem-example

Explanation

Summary

While other ESP8266 firmwares (NodeMCU, smart.js and frankenstein) already come with a filesystem it is great to see that the Arduino/ESP8266 environment soon will be blessed with this important feature. It is easy to apply and a huge improvement over the very low-level EEPROM persistence interface. Directories let you structure your files and you can freely create new files, delete or rename them.
Compared to the NodeMCU firmware you have worry how you can get the files onto the device. If you want to run a web server you cannot just transfer the html, css and js files from your PC to the ESP8266. There is no interface for such an operation yet. But you could store them on another webserver and store/update them in flash when the device connects to WiFi.