add readme

Author: Piotr Suwała

README.md

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+# Prerequisites
+
+1. XCode
+2. Android Studio
+
+# How to run the project
+
+1. Clone the repository.
+2. Run `./downloadAndInsertOpenCV.sh`.
+3. Download manually the Android pack from https://opencv.org/releases.html (version 3.4.1).
+4. Unzip the package.
+5. Import OpenCV to Android Studio, From File -> New -> Import Module, choose sdk/java folder in the unzipped opencv archive.
+6. Update build.gradle under imported OpenCV module to update 4 fields to match your project build.gradle a) compileSdkVersion b) buildToolsVersion c) minSdkVersion and d) targetSdkVersion.
+7. Add module dependency by Application -> Module Settings, and select the Dependencies tab. Click + icon at bottom, choose Module Dependency and select the imported OpenCV module. For Android Studio v1.2.2, to access to Module Settings : in the project view, right-click the dependent module -> Open Module Settings.
+8. `react-native run-ios` or `react-native run-android`.