esp32: Add build instructions.

Author: Daniel Campora

esp32/README.md

@@ -6,3 +6,27 @@ path must be specified via the ESP_IDF_PATH variable. See the Makefile for detai
 
 The modified Espressif IDF that we use to build this port can be found in:
 https://github.com/pycom/pycom-esp-idf
+
+Build instructions
+------------------
+
+First build the mpy-cross compiler:
+
+    $ cd ../mpy-cross
+    $ make all
+
+After that, build the ESP32 port for one of Pycom boards (first the bootloader, then the app):
+
+    $ cd ../esp32
+    $ make BOARD=LOPY -j5 TARGET=boot
+    $ make BOARD=LOPY -j5 TARGET=app clean
+
+Flash the board (connect P2 to GND and reset before starting):
+
+    $ make BOARD=LOPY flash
+
+Using frozen modules
+--------------------
+
+Place all the python scripts that you'd like to be frozen into the flash memory of the board inside
+the 'frozen' folder in the esp32 directory. Then build as indicated before.