Add markdown docs to docs/ dir for community access

Author: cbullinger

docs/README.md

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+# Realm SDK for Java
+Use the Realm SDK for Java to develop Android apps in Java or Kotlin.
+To develop multiplatform apps using Kotlin Multiplatform (KMP), refer to the
+Kotlin SDK.
+
+## SDK in Maintenance Mode
+This SDK is in best-effort maintenance mode and **no longer receives
+new development or non-critical bug fixes**. To develop your app with new
+features, use the Kotlin SDK. You can use the Java SDK with the Kotlin SDK.
+
+## Develop Apps with the SDK
+Use the SDK's open-source database - Realm - as an object store on the device.
+
+### Install the Java SDK
+Use the Gradle build system to
+install the Java SDK in your project.
+
+### Define an Object Schema
+Use Java or Kotlin to idiomatically define an object schema.
+
+### Open a Database
+The SDK's database - Realm - stores objects in files on your device.
+Or you can open an in-memory database which does not create a file.
+To get started reading and writing data,
+configure and open a database.
+
+### Read and Write Data
+Create, read, update, and
+delete objects from the database.
+Use Android-native queries to filter data.
+
+### React to Changes
+Live objects mean that your data is always up-to-date.
+You can register a notification handler to watch for changes and perform some
+logic, such as updating your UI.
+