Add note about needing 0.4 esp8266fs uploader

Update the docs to point to the later version of the 8266 sketch data
uploader and inform existing users of the need to upgrade.

Fixes esp8266#5845

Author: earlephilhower

doc/filesystem.rst

@@ -107,11 +107,18 @@ Uploading files to file system
 menu item to *Tools* menu for uploading the contents of sketch data
 directory into ESP8266 flash file system.
 
--  Download the tool: https://github.com/esp8266/arduino-esp8266fs-plugin/releases/download/0.3.0/ESP8266FS-0.3.0.zip.
+**Warning**: Due to the move from the obsolete esptool-ck.exe to the
+supported esptool.py upload tool, upgraders from pre 2.5.1 will need to
+update the ESP8266FS tool referenced below to 0.4.0 or later.  Prior versions
+will fail with a "esptool not found" error because they don't know how to
+use esptool.py.
+
+-  Download the tool: https://github.com/esp8266/arduino-esp8266fs-plugin/releases/download/0.4.0/ESP8266FS-0.4.0.zip
 -  In your Arduino sketchbook directory, create ``tools`` directory if
    it doesn't exist yet
 -  Unpack the tool into ``tools`` directory (the path will look like
    ``<home_dir>/Arduino/tools/ESP8266FS/tool/esp8266fs.jar``)
+   If upgrading, overwrite the existing JAR file with the newer version.
 -  Restart Arduino IDE
 -  Open a sketch (or create a new one and save it)
 -  Go to sketch directory (choose Sketch > Show Sketch Folder)