Add ReadMe for compiler/android-tests

compiler/android-tests/ReadMe.md

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+# Codegen tests on Android
+
+This module runs codegen box tests (`compiler/testData/codegen/box`) on Android. It does so by compiling all of tests,
+except the excluded ones, in one big Android project and running it as an app on an emulator, which is downloaded during
+the first run of the tests. See which tests are excluded in `CodegenTestsOnAndroidGenerator`, but mainly those are the
+ones annotated with `// IGNORE_BACKEND: ANDROID`, those having Java source files, or using advanced Kotlin/JVM features.
+
+Run the tests via Gradle:
+
+```
+./gradlew :compiler:android-tests:test
+```
+
+**Make sure your JAVA_HOME points to a JDK 1.8 installation**, otherwise, you'll get an exception, such as
+`java.lang.ClassNotFoundException: javax.xml.bind.annotation.XmlSchema`.