Add basic README (#24)

README.md

@@ -0,0 +1,46 @@
+# divviup-android
+An Android client library for the [Distributed Aggregation Protocol][DAP].
+
+[DAP]: https://datatracker.ietf.org/doc/draft-ietf-ppm-dap/
+
+## Protocol Versions and Release Branches
+
+The following versions of the DAP protocol are supported by different branches
+and releases.
+
+| Package version | Git branch | Protocol version | Conformant? | Status |
+|-----------------|------------|------------------|-------------|--------|
+| 0.1.0 (forthcoming) | `main` | [`draft-ietf-ppm-dap-07`][draft-07] | Yes | Supported |
+
+[draft-07]: https://datatracker.ietf.org/doc/draft-ietf-ppm-dap/07/
+
+## Usage
+
+Note that no published releases are available yet, so you must build this
+library from source for now. Add the AAR file to your project. Construct a
+`Client` from your DAP task's parameters, and use it to send report as follows.
+(Note that this should be done off the main thread)
+
+```java
+import android.util.Log;
+
+import org.divviup.android.Client;
+import org.divviup.android.TaskId;
+
+import java.net.URI;
+
+public class MyRunnable implements Runnable {
+    public void run() {
+        try {
+            URI leaderEndpoint = new URI("https://<your leader here>/");
+            URI helperEndpoint = new URI("https://<your helper here>/");
+            TaskId taskId = TaskId.parse("<your DAP TaskId here>");
+            long timePrecisionSeconds = <your time precision here>;
+            Client<Boolean> client = Client.createPrio3Count(leaderEndpoint, helperEndpoint, taskId, timePrecisionSeconds);
+            client.sendMeasurement(<your measurement here>);
+        } catch (Exception e) {
+            Log.e("MyRunnable", "upload failed", e);
+        }
+    }
+}
+```