Need to document SPIFFS Data Uploader update steps for 2.5.0 to 2.5.0++

[earlephilhower]

The esptool.py merge needs an update to the SPIFFS data uploader JAVA.

The new version was merged and a release generated in https://github.com/esp8266/arduino-esp8266fs-plugin , but we still need @igrr or @me-no-dev to do an official build of the esptool and put it in the release 0.4.

Once that's done, need to update the docs to note the incompatibilty of older revs which expect esptool.exe.