doc: Update documentation for Android builds

depends/README.md

@@ -34,8 +34,11 @@ Common `host-platform-triplet`s for cross compilation are:
 - `riscv32-linux-gnu` for Linux RISC-V 32 bit
 - `riscv64-linux-gnu` for Linux RISC-V 64 bit
 - `s390x-linux-gnu` for Linux S390X
+- `armv7a-linux-android` for Android ARM 32 bit
+- `aarch64-linux-android` for Android ARM 64 bit
+- `x86_64-linux-android` for Android x86 64 bit
 
-The paths are automatically configured and no other options are needed.
+The paths are automatically configured and no other options are needed unless targeting [Android](../doc/build-android.md).
 
 ### Install the required dependencies: Ubuntu & Debian
 

doc/README.md

@@ -45,6 +45,7 @@ The following are developer notes on how to build Bitcoin Core on your native pl
 - [FreeBSD Build Notes](build-freebsd.md)
 - [OpenBSD Build Notes](build-openbsd.md)
 - [NetBSD Build Notes](build-netbsd.md)
+- [Android Build Notes](build-android.md)
 
 Development
 ---------------------

doc/build-android.md

@@ -0,0 +1,44 @@
+# Android Build Guide
+
+This guide describes how to build and package the `bitcoin-qt` GUI for Android on Linux and macOS.
+
+
+## Dependencies
+
+Before proceeding with an Android build one needs to get the [Android SDK](https://developer.android.com/studio) and use the "SDK Manager" tool to download the NDK and one or more "Platform packages" (these are Android versions and have a corresponding API level).
+
+Qt 6.7.3 used in depends supports Android NDK version is [r26b (26.1.10909125)](https://github.com/android/ndk/wiki/Changelog-r26).
+
+In order to build `ANDROID_API_LEVEL` (API level corresponding to the Android version targeted, e.g. Android 9.0 Pie is 28 and its "Platform package" needs to be available) needs to be set.
+
+Qt 6.7.3 used in depends supports API levels from 26 to 34.
+
+When building [depends](../depends/README.md), additional variables `ANDROID_SDK` and `ANDROID_NDK` need to be set.
+
+This is an example command for a default build with no disabled dependencies:
+
+    gmake HOST=aarch64-linux-android ANDROID_SDK=/home/user/Android/Sdk ANDROID_NDK=/home/user/Android/Sdk/ndk/26.1.10909125 ANDROID_API_LEVEL=34
+
+
+## Building and packaging
+
+After the depends are built configure, build and create an Android Application Package (APK) as follows (based on the example above):
+
+```bash
+cmake -B build --toolchain depends/aarch64-linux-android/toolchain.cmake
+cmake --build build --target apk_package  # Use "-j N" for N parallel jobs.
+```
+
+The APKs will be available in the following directory:
+```bash
+$ tree build/src/qt/android/build/outputs/apk
+build/src/qt/android/build/outputs/apk
+├── debug
+│   ├── android-debug.apk
+│   └── output-metadata.json
+└── release
+    ├── android-release-unsigned.apk
+    └── output-metadata.json
+
+3 directories, 4 files
+```