doc: add guide on working with system.img

README.md

@@ -161,6 +161,17 @@ Please note that to use 'gradle flash', your host machine must be connectted to
 
 </details>
 
+<details>
+  <summary>working with system.img</summary>
+
+First enable **hacking mode** by setting `bHackingMode = true` in file `build.gradle.kts`, then
+```bash
+cp <your_system_image> system.img
+./gradlew unpack
+```
+You get `system.img.unsparse`, that's a plain ext4 filesystem data.
+
+</details>
 
 <details>
   <summary>How to disable AVB verification</summary>