Add readme

README.md

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+This Android library offers a **standardized way** for app developers to
+
+- **provide chess engines to other chess apps**
+
+- **use provided chess engines in other chess apps**
+
+
+# Engine authors: how to provide a chess engine using this library #
+
+  * See the StockfishChessEngine directory for an example of an app which provides Stockfish to other apps
+
+# GUI authors: how to support the open exchange format #
+
+  * Import the library. Android Studio: File - New - Import Module (select the chessEngineSupportLibrary directory)
+  * use something like:
+```java
+EngineResolver resolver = new EngineResolver(context);
+List<Engine> engines = resolver.resolveEngines();
+```
+> engines is now a list of ChessEngines for the current target. E.g.:
+```java
+TextView text = findViewById(R.id.exampleText);
+for (ChessEngine engine : engines) {
+    text.append(engine.getName() + ": " + engine.getEnginePath() + "\n");
+}
+```
+> you can directly execute the engine from the engine path