Document proot binary placement

app/src/main/jniLibs/README.md

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+# proot binaries (not committed yet)
+
+Android 10+ only lets an app execute files from its install-time native library directory.
+To enable `proot install` / `proot <cmd>` in the console, place these two files here before building:
+
+```
+app/src/main/jniLibs/arm64-v8a/libproot.so          # proot executable (Termux build, GPL-2.0)
+app/src/main/jniLibs/arm64-v8a/libproot-loader.so   # the loader that matches that proot build
+```
+
+Source: https://github.com/termux/proot (build with termux-packages, or use a static build).
+The proot binary and its loader must come from the same build.
+Until they are added, `proot status` prints `binary=false`, and the `termux <cmd>` command
+(Termux + proot-distro) is the working route to a Linux environment.