Minor FS documentation change from esp8266#2904

Trivial addition to filesystem upload documentation.

Supercedes esp8266#2904 which cannot merge due to file renames and massive
changes to the filesystem.rst file since it was pushed.

Author: earlephilhower

doc/filesystem.rst

@@ -185,6 +185,8 @@ use esptool.py.
 -  Create a directory named ``data`` and any files you want in the file
    system there
 -  Make sure you have selected a board, port, and closed Serial Monitor
+-  If your board requires you to press a button (or other action) to enter
+   bootload mode for flashing a sketch, do that now.
 -  Select Tools > ESP8266 Sketch Data Upload. This should start
    uploading the files into ESP8266 flash file system. When done, IDE
    status bar will display ``SPIFFS Image Uploaded`` message.