added how to build image into README.md

README.md

@@ -1,3 +1,47 @@
 # FruitOS
 
 This repository contains files to build FruitOS image.
+
+
+## How to build FruitOS image
+
+1. Install required software:
+
+	```
+	apk update
+	apk add make git
+	```
+
+2. Initialize and update git submodules
+
+	```
+	git submodule init
+	git submodule update
+	```
+
+3. Ensure the following entries available in `/etc/apk/repositories`:
+
+	```
+	http://dl-cdn.alpinelinux.org/alpine/v3.6/main
+	http://dl-cdn.alpinelinux.org/alpine/v3.6/community
+	http://dl-cdn.alpinelinux.org/alpine/edge/testing
+	```
+
+4. Copy FruitOS private key into submodule **apks** to sign software packages:
+
+	```
+	cp fruit-apk-key.rsa apks/
+	```
+
+5. Build software packages and FruitOS image for Raspberry Pi 2, 3, and compute module 3:
+
+	```
+	make
+	```
+    If everything goes well, then you will find a FruitOS image file: `disk.img`.
+
+    If you want to build an image for Raspberry Pi Zero, Zero-W, 1, and compute module, then you have to invoke:
+
+	```
+	MACHINE=rpi make
+	```