doc: Add minimum supported Android NDK version

Author: hebasto

doc/build-android.md

@@ -7,6 +7,9 @@ This guide describes how to build and package the `bitcoin-qt` GUI for Android o
 ## 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).
+
+The minimum supported Android NDK version is [r21](https://github.com/android/ndk/wiki/Changelog-r21).
+
 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.
 
 API levels from 24 to 29 have been tested to work.