Add NDK version in README.md

[liveHarshit]

Build failed in the newer version of NDK 17c due to no toolchains for mips64el-linux-android. Working fine with version 16b. It should mention in the readme file to use an older version of NDK or provide a link to download a zip file for toolchain mips64el-linux-android.

[mariobehling]

It would be great if you could update the app to use the newer version instead.