Add readme

Author: edgchen1

mobile/examples/model_tester/ios/ModelTester/readme.md

@@ -0,0 +1,11 @@
+This is a basic app that runs an arbitrary ONNX model. It can measure the model's performance.
+
+# Set Up Instructions
+
+Copy an ONNX model to `model/model.onnx`.
+
+Install the CocoaPods dependencies. From this directory, run `pod install`.
+
+Open the xcworkspace file. From this directory, run `open ModelTester.xcworkspace`.
+
+Build and run the ModelTester app from Xcode.