espressif: Add espressif's readme file to the docs

Add readme-espressif.md documentation for Espressif port.

Signed-off-by: Almir Okato <almir.okato@espressif.com>

docs/readme-espressif.md

@@ -0,0 +1,70 @@
+# Building and using MCUboot with Espressif's chips
+
+The Espressif port is build on top of ESP-IDF HAL, therefore it is required in order to build MCUboot for Espressif SoCs.
+
+Documentation about the MCUboot bootloader design, operation and features can be found in the [design document](design.md).
+
+## SoC support availability
+
+The current port is available for use in the following SoCs within the OSes:
+- ESP32
+    - Zephyr RTOS - _WIP_
+    - NuttX
+- ESP32-S2
+    - Zephyr RTOS - _WIP_
+    - NuttX - _WIP_
+
+## Installing Requirements and Dependencies
+
+1. Install additional packages required for development with MCUboot:
+
+```
+  cd ~/mcuboot  # or to your directory where mcuboot is cloned
+  pip3 install --user -r scripts/requirements.txt
+```
+
+2. Update the submodules needed by the Espressif port. This may take a while.
+
+```
+git submodule update --init --recursive --checkout boot/espressif/hal/esp-idf
+```
+
+3. Next, get the mbedtls submodule required by MCUboot.
+```
+git submodule update --init --recursive ext/mbedtls
+```
+
+4. Now we need to install IDF dependencies and set environment variables. This step may take some time:
+```
+cd boot/espressif/hal/esp-idf
+./install.sh
+. ./export.sh
+cd ../..
+```
+
+## Building the bootloader itself
+
+The MCUboot Espressif port bootloader is built using the toolchain and tools provided by ESP-IDF. Additional configuration related to MCUboot features and slot partitioning may be made using the `bootloader.conf`.
+
+**Note:** Replace `<target>` with the target ESP32 family (like `esp32`, `esp32s2` and others).
+
+1. Compile and generate the ELF:
+
+```
+cmake -DCMAKE_TOOLCHAIN_FILE=tools/toolchain-<target>.cmake -DMCUBOOT_TARGET=<target> -B build -GNinja
+cmake --build build/
+```
+
+2. Convert the ELF to the final bootloader image, ready to be flashed:
+
+```
+esptool.py --chip <target> elf2image --flash_mode dio --flash_freq 40m -o build/mcuboot_<target>.bin build/mcuboot_<target>.elf
+```
+
+3. Flash MCUboot in your board:
+
+```
+esptool.py -p <PORT> -b <BAUD> --before default_reset --after hard_reset --chip <target> write_flash --flash_mode dio --flash_size detect --flash_freq 40m 0x1000 build/mcuboot_<target>.bin
+```
+
+You may adjust the port `<PORT>` (like `/dev/ttyUSB0`) and baud rate `<BAUD>` (like `2000000`) according to the connection with your board.