Add information to the README about how GABE works

README.md

@@ -7,7 +7,7 @@ build environment to perform Gradle builds when running natively on an Android d
 
 This is done through an Android service which leverages a
 [rootfs](https://github.com/godotengine/android-editor-buildenv-rootfs) and [proot](https://github.com/termux/proot) to
-set up a build environment able to perform the Gradle builds.
+set up a build environment able to perform the Gradle builds. See the **How GABE works** section to learn more.
 
 ## Getting the app
 
@@ -40,3 +40,37 @@ gradlew.bat build
 ```
 
 The generated binaries can be found under the `app/build/outputs/apk` directory.
+
+## How GABE works
+
+Gradle isn't designed to run on Android, but it _can_ run on normal Linux systems.
+Android does use the Linux kernel, but it doesn't provide a standard Linux environment,
+and so can't run Gradle out-of-the-box.
+
+One of the first steps when using GABE is to install a "rootfs", which is, in fact,
+a Linux rootfs (based on Alpine Linux) which provides that standard Linux environment.
+Our rootfs also includes other dependencies that Gradle needs, namely Java and the
+Android SDK, built in a special way to run on Android itself.
+
+GABE uses a tool called [proot](https://proot-me.github.io/), which allows executing
+programs (like Gradle) inside of a rootfs, such that the programs see the rootfs as
+the full system environment.
+
+So, from the perspective of Gradle, it thinks it's running within Alpine Linux!
+
+GABE exposes an Android service that other applications (like Godot) can send messages
+to, in order to trigger it to run Gradle on a given project.
+
+## Why a companion app?
+
+There are several reasons for making GABE as a companion app, rather than building
+its functionality directly into the Godot editor itself:
+
+- **Licensing:** Godot uses the MIT license, but proot is licensed under the GPL.
+  By keeping them separate and using message passing, we don't have to worry about
+  the "viral" nature of the GPL affecting Godot. This is also why GABE uses the GPL.
+- **Updates:** Keeping GABE separate allows us to update GABE at its own pace, and
+  not be tied to Godot's release cycle. It also allows us to potentially support
+  multiple Godot versions from a single version of GABE.
+- **Size:** Not all developers will want to do Gradle builds, so they shouldn't
+  have to pay for the additional space that it takes on their device.