add android NDK event loop (#1556)

* add android NDK event loop

* add Android build documentation & cargo-apk to CI

Co-authored-by: David Craven <david@craven.ch>

README.md

@@ -76,3 +76,22 @@ Building a binary will yield a `.js` file. In order to use it in an HTML file, y
   the element of the `<canvas>` element (in the example you would retrieve it via `document.getElementById("my_id")`).
   More information [here](https://kripken.github.io/emscripten-site/docs/api_reference/module.html).
 - Make sure that you insert the `.js` file generated by Rust after the `Module` variable is created.
+
+#### Android
+
+This library makes use of the [ndk-rs](https://github.com/rust-windowing/android-ndk-rs) crates, refer to that repo for more documentation.
+
+Running on an Android device needs a dynamic system library, add this to Cargo.toml:
+```toml
+[[example]]
+name = "request_redraw_threaded"
+crate-type = ["cdylib"]
+```
+
+And add this to the example file to add the native activity glue:
+```rust
+#[cfg(target_os = "android")]
+ndk_glue::ndk_glue!(main);
+```
+
+And run the application with `cargo apk run --example request_redraw_threaded`