Filesystem Image Creation documentation

[avanti1]

I spent a day trying to get the "littlefs_create_partition_image" feature working. I finally found out that if you actually want it to move your files to flash, you need to add the string "FLASH_IN_PROJECT" as a third parameter. I think it would help people if this were noted.

It should look something like this:
littlefs_create_partition_image(littlefs /esp/littlefsfiles FLASH_IN_PROJECT)

[BrianPugh]

So sorry about that! I just updated the README in 28639dd to reflect that most people should probably be adding FLASH_IN_PROJECT to their littlefs_create_partition_image call. Thanks for reporting this!