Add chapter about firmware files and flash address

Had to slightly tweak the Markdown headers due to
https://github.com/mkdocs/mkdocs/issues/770 :(.

Fixes #990.

docs/en/flash.md

@@ -2,13 +2,7 @@ Adafruit provides a really nice [firmware flashing tutorial](https://learn.adafr
 
 !!! note "Note:"
 
-    Keep in mind that the ESP8266 needs to be put into flash mode before you can flash a new firmware!
+    Keep in mind that the ESP8266 needs to be [put into flash mode](#putting-device-into-flash-mode) before you can flash a new firmware!
-
-To enable ESP8266 firmware flashing GPIO0 pin must be pulled low before the device is reset. Conversely, for a normal boot, GPIO0 must be pulled high or floating.
-
-If you have a [NodeMCU dev kit](https://github.com/nodemcu/nodemcu-devkit-v1.0) then you don't need to do anything, as the USB connection can pull GPIO0 low by asserting DTR and reset your board by asserting RTS.
-
-If you have an ESP-01 or other device without built-in USB, you will need to enable flashing yourself by pulling GPIO0 low or pressing a "flash" switch.
 
 ## esptool
 > A cute Python utility to communicate with the ROM bootloader in Espressif ESP8266. It is intended to be a simple, platform independent, open source replacement for XTCOM.
@@ -30,12 +24,33 @@ Source: [https://github.com/nodemcu/nodemcu-flasher](https://github.com/nodemcu/
 
 Supported platforms: Windows
 
+## Putting Device Into Flash Mode
+
+To enable ESP8266 firmware flashing GPIO0 pin must be pulled low before the device is reset. Conversely, for a normal boot, GPIO0 must be pulled high or floating.
+
+If you have a [NodeMCU dev kit](https://github.com/nodemcu/nodemcu-devkit-v1.0) then you don't need to do anything, as the USB connection can pull GPIO0 low by asserting DTR and reset your board by asserting RTS.
+
+If you have an ESP-01 or other device without built-in USB, you will need to enable flashing yourself by pulling GPIO0 low or pressing a "flash" switch.
+
+## Which Files To Flash
+
+If you build your firmware with the [cloud builder or the Docker image](build.md), or any other method that produces a *combined binary*, then you can flash that file directly to address 0x00000.
+
+Otherwise, if you built your own firmware from source code:
+
+- `bin/0x00000.bin` to 0x00000
+- `bin/0x10000.bin` to 0x10000
+
+Also, in some special circumstances, you may need to flash `blank.bin` or `esp_init_data_default.bin` to various addresses on the flash (depending on flash size and type), see [below](#upgrading-from-sdk-09x-firmware).
+
+If upgrading from [SPIFFS](https://github.com/pellepl/spiffs) version 0.3.2 to 0.3.3 or later, or after flashing any new firmware (particularly one with a much different size), you may need to run [`file.format()`](modules/file.md#fileformat) to re-format your flash filesystem. You will know if you need to do this if your flash files disappeared, or if they exist but seem empty, or if data cannot be written to new files.
+
 ## Upgrading from SDK 0.9.x Firmware
-If you flash a recent NodeMCU firmware for the first time, it's advisable that you get all accompanying files right. A typical case that often fails is when a module is upgraded from a 0.9.x firmware to the latest version built from the [NodeMCU build service](http://nodemcu-build.com). It might look like the brand new firmware is broken, but the reason for the missing Lua prompt is related to the big jump in SDK versions: Vendor Espressif changed the `esp_init_data_default.bin` for their devices along the way with the [SDK 1.4.0 release](http://bbs.espressif.com/viewtopic.php?f=46&t=1124). So things break when a NodeMCU firmware with SDK 1.4.0 or above is flashed to a module which contains old init data from a previous SDK.
+If you flash a recent NodeMCU firmware for the first time, it's advisable that you get all accompanying files right. A typical case that often fails is when a module is upgraded from a 0.9.x firmware to the latest version built from the [NodeMCU build service](http://nodemcu-build.com). It might look like the brand new firmware is broken, but the reason for the missing Lua prompt is related to the big jump in SDK versions: Espressif changed the `esp_init_data_default.bin` for their devices along the way with the [SDK 1.4.0 release](http://bbs.espressif.com/viewtopic.php?f=46&t=1124). So things break when a NodeMCU firmware with SDK 1.4.0 or above is flashed to a module which contains old init data from a previous SDK.
 
 Download a recent SDK release, e.g. [esp_iot_sdk_v1.4.0_15_09_18.zip](http://bbs.espressif.com/download/file.php?id=838) or later and extract `esp_init_data_default.bin` from there. *Use this file together with the new firmware during flashing*.
 
-### esptool
+**esptool**
 
 For [esptool](https://github.com/themadinventor/esptool) you specify another file to download at the command line.
 ```
@@ -46,7 +61,7 @@ esptool.py write_flash 0x00000 <nodemcu-firmware>.bin 0x7c000 esp_init_data_defa
 
     The address for `esp_init_data_default.bin` depends on the size of your module's flash. ESP-01, -03, -07 etc. with 512 kByte flash require `0x7c000`. Init data goes to `0x3fc000` on an ESP-12E with 4 MByte flash.
 
-### NodeMCU Flasher
+**NodeMCU Flasher**
 
 The [NodeMCU Flasher](https://github.com/nodemcu/nodemcu-flasher) will download init data using a special path:
 ```
@@ -55,7 +70,7 @@ INTERNAL://DEFAULT
 
 Replace the provided (old) `esp_init_data_default.bin` with the one extracted above and use the flasher like you're used to.
 
-### References
+**References**
 
 * [2A-ESP8266__IOT_SDK_User_Manual__EN_v1.5.pdf, Chapter 6](http://bbs.espressif.com/viewtopic.php?f=51&t=1024)
 * [SPI Flash ROM Layout (without OTA upgrades)](https://github.com/esp8266/esp8266-wiki/wiki/Memory-Map#spi-flash-rom-layout-without-ota-upgrades)