Add documentation of development environment setup

CONTRIBUTING.md

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+# Contributing to divviup-android
+
+## Prerequisites
+
+Building this project requires a Java JDK (17 or newer), the Android SDK, Rust,
+and Docker to be installed. It is highly recommended to use [Android
+Studio](https://developer.android.com/studio).
+
+### Android Studio
+
+Installing Android Studio will provide both a Java JDK and the Android SDK.
+Android Studio is the recommended way to install and manage Android SDK
+components. Refer to the documentation on [compatible versions of the Android
+Gradle plugin and Android Studio][compat-docs] to determine what version to
+install. The Android Gradle plugin's version is set in `/build.gradle.kts`.
+
+[compat-docs]:
+    https://developer.android.com/studio/releases#android_gradle_plugin_and_android_studio_compatibility
+
+### Android SDK
+
+An Android SDK must be installed, along with several additional components. SDK
+components can be managed from Android Studio via Tools -> SDK Manager, or on
+the command line via `$ANDROID_HOME/cmdline-tools/latest/bin/sdkmanager`.
+
+Install the version of "Android SDK Build-Tools" that matches the `compileSdk`
+version in `/divviup/build.gradle.kts`. Install the version of the NDK that
+matches `ndkVersion` in `/divviup/build.gradle.kts`. Install the "Android
+Emulator".
+
+Additionally, you will need to set up a virtual emulator device and download a
+system image for it. This can be done via Tools -> Device Manager in Android
+Studio or `$ANDROID_HOME/cmdline-tools/latest/bin/avdmanager` on the command
+line. Use a system image for a recent API level and the processor architecture
+matching your host computer for day-to-day development.
+
+When making major FFI changes or adding support for a new architecture, you may
+want to return to the device manager and create new virtual devices with
+different architectures for additional testing. Note that there is a significant
+performance difference between using a processor architecture that matches the
+host (which can be virtualized) versus using a dissimilar processor architecture
+(which must be emulated).
+
+### Rust
+
+Additional files must be installed to cross-compile for Android platforms. Run
+the following command to download target-specific copies of the standard
+library.
+
+```sh
+rustup target add armv7-linux-androideabi aarch64-linux-android i686-linux-android x86_64-linux-android
+```
+
+### Docker
+
+[Docker][docker-install] is required for host-based integration tests against
+Janus, to confirm that reports are correctly formed, and DAP API requests are
+correct. Alternative container runtimes may be used in lieu of Docker, see the
+[Testcontainers documentation][testcontainers-docker] for details.
+
+[docker-install]: https://docs.docker.com/desktop/
+[testcontainers-docker]: https://java.testcontainers.org/supported_docker_environment/
+
+## Building and Testing
+
+All compilation and testing is managed by Gradle targets, which can be used from
+both Android Studio and the command line. Right click on a project or a test
+folder in Android Studio to run or debug it. Running the sample app launches the
+emulator within the Android Studio UI. Any `test` directory runs on the host's
+JDK, while any `androidTest` must run in an emulator or a connected device in
+developer mode. Gradle can be invoked on the command line as follows.
+
+```sh
+# Runs host-based tests.
+./gradlew check
+
+# Runs device-based tests. An emulator must already be running.
+./gradlew connectedCheck
+
+# Build the AAR.
+./gradlew :divviup:assemble
+
+# Build the sample app's APK.
+./gradlew :sampleapp:assemble
+```
+
+Note that the CI only runs `./gradlew check`, and not `./gradlew
+connectedCheck`, because of the difficulty of accessing an Android emulator from
+CI runners.
+
+## Releases
+
+See [internal
+documentation](https://github.com/divviup/docs/blob/main/janus/releasing-janus.md#generating-a-release-1).