Add full Android build example command and instructions on getting SDK/NDK

Author: icota

depends/README.md

@@ -33,9 +33,14 @@ Common `host-platform-triplets` for cross compilation are:
 - `aarch64-linux-android` for Android ARM 64 bit
 
 The paths are automatically configured and no other options are needed unless targeting Android.
-In that case one needs to set `ANDROID_API_LEVEL` and `ANDROID_TOOLCHAIN_BIN`:
+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).
+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) and `ANDROID_TOOLCHAIN_BIN` (path to toolchain binaries depending on the platform the build is being performed on) need to be set.
+If the build includes Qt, environment variables `ANDROID_SDK` and `ANDROID_NDK` need to be set as well but can otherwise be omitted.
+This is an example command for a default build with no disabled dependencies:
+
+    ANDROID_SDK=/home/user/Android/Sdk ANDROID_NDK=/home/user/Android/Sdk/ndk-bundle make HOST=aarch64-linux-android ANDROID_API_LEVEL=28 ANDROID_TOOLCHAIN_BIN=/home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin
+
 
-    make HOST=aarch64-linux-android ANDROID_API_LEVEL=28 ANDROID_TOOLCHAIN_BIN=/home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin NO_QT=1 NO_WALLET=1
 
 ### Install the required dependencies: Ubuntu & Debian