Add howto compile docs

docs/howto-compile.md

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+# How to compile AirGradient firmware on Arduino IDE
+
+## Prequisite
+
+Arduino IDE version 2.x
+
+## Steps for ESP32C3 based board (ONE and OpenAir Model)
+
+1. Install esp32 board on board manager with version **2.0.17**
+
+![board manager](images/esp32-board.png)
+
+2. Install AirGradient library on library manager using the latest version (at the time of writing, its 3.2.0)
+
+![Aigradient Library](images/ag-lib.png)
+
+3. Plug AirGradient monitor
+4. On tools tab, follow settings below
+
+```
+Board ➝ ESP32C3 Dev Module
+USB CDC On Boot ➝ Enabled
+CPU Frequency ➝ 160MHz (WiFi)
+Core Debug Level ➝ None (or choose as needed)
+Erase All Flash Before Sketch Upload ➝ Enabled (or choose as needed)
+Flash Frequency ➝ 80MHz
+Flash Mode ➝ QIO
+Flash Size ➝ 4MB
+JTAG Adapter ➝ Disabled
+Partition Scheme ➝ Minimal SPIFFS (1.9MB APP with OTA/190KB SPIFFS)
+Upload Speed ➝ 921600
+```
+
+![Compile Settings](images/settings.png)
+
+5. Compile
+
+## Steps for ESP8266 based board (DIY model)
+
+1. Add esp8266 board by adding http://arduino.esp8266.com/stable/package_esp8266com_index.json into Additional Board Manager URLs field.
+2. Install esp8266 board on board manage with version **3.1.2**
+
+![board manager](images/esp8266-board.png)
+
+3. Plug AirGradient monitor
+4. Set board to `LOLIN(WEMOS) D1 R2 & mini`, let other settings to default
+5. Compile
+
+## Possible Issues
+
+### Linux (Debian)
+
+ModuleNotFoundError: No module named ‘serial’
+
+Make sure python pyserial module installed globally in the environment by executing: 
+
+`$ pip install pyserial`