copy readme to plugin dir to maybe make npm repo happy

plugin/README.md

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+# Play Install Referrer Library wrapper for React Native
+
+<table>
+    <tr>
+        <td align="left">Supported platforms:</td>
+        <td align="left"><img src="https://images-fe.ssl-images-amazon.com/images/I/21EctgvtXUL.png" width="16"></td>
+    </tr>
+    <tr>
+        <td align="left">Current version:</td>
+        <td align="left"><a href=../../releases/tag/v1.1.3><b>1.1.3</b></a></td>
+    </tr>
+    <tr>
+        <td align="left">Troubles?</td>
+        <td align="left"><a href="../../issues/new"><b>Report an issue</b></a></td>
+    </tr>
+</table>
+
+**react-native-play-install-referrer** is a simple wrapper around Google's [Play Install Referrer Library](https://developer.android.com/google/play/installreferrer/library) which offers basic functionality of obtaining Android referrer information from React Native app.
+
+More information about Play Install Referrer API can be found in [official Google documentation](https://developer.android.com/google/play/installreferrer/igetinstallreferrerservice).
+
+Version of native Play Install Referrer Library which is being used inside of latest **react-native-play-install-referrer** plugin version is [2.1](https://mvnrepository.com/artifact/com.android.installreferrer/installreferrer/2.1).
+
+## Add plugin to your app
+
+**react-native-play-install-referrer** plugin is hosted on [npm repo](https://www.npmjs.com/package/react-native-play-install-referrer) and can be added from there.
+
+**Yarn**:
+
+```
+yarn add react-native-play-install-referrer
+```
+
+**npm**:
+
+```
+npm install --save react-native-play-install-referrer
+```
+
+## Usage
+
+In order to obtain install referrer details, call **getInstallReferrerInfo** static method of **PlayInstallReferrer** class:
+
+```js
+import { PlayInstallReferrer } from 'react-native-play-install-referrer';
+
+PlayInstallReferrer.getInstallReferrerInfo((installReferrerInfo, error) => {
+  if (!error) {
+    console.log("Install referrer = " + installReferrerInfo.installReferrer);
+    console.log("Referrer click timestamp seconds = " + installReferrerInfo.referrerClickTimestampSeconds);
+    console.log("Install begin timestamp seconds = " + installReferrerInfo.installBeginTimestampSeconds);
+    console.log("Referrer click timestamp server seconds = " + installReferrerInfo.referrerClickTimestampServerSeconds);
+    console.log("Install begin timestamp server seconds = " + installReferrerInfo.installBeginTimestampServerSeconds);
+    console.log("Install version = " + installReferrerInfo.installVersion);
+    console.log("Google Play instant = " + installReferrerInfo.googlePlayInstant);
+  } else {
+    console.log("Failed to get install referrer info!");
+    console.log("Response code: " + error.responseCode);
+    console.log("Message: " + error.message);
+  }
+});
+```
+
+If successfully obtained, map with content of install referrer information will be delivered into callback method as first parameter. From that map, you can get following install referrer details:
+
+- Install referrer string value (**installReferrer** key).
+- Timestamp of when user clicked on URL which redirected him/her to Play Store to download your app (**referrerClickTimestampSeconds** key).
+- Timestamp of when app installation on device begun (**installBeginTimestampSeconds** key).
+- Server timestamp of when user clicked on URL which redirected him/her to Play Store to download your app (**referrerClickTimestampServerSeconds** key).
+- Server timestamp of when app installation on device begun (**installBeginTimestampServerSeconds** key).
+- Original app version which was installed (**installVersion** key).
+- Information if your app's instant version (if you have one) was launched in past 7 days (**googlePlayInstant** key).
+
+You should first check if second parameter in the callback - **error** is **null** or not. If not, for some reason reading of install referrer details failed. In case no error is reported, install referrer information should be delivered into the first map parameter of the callback method.
+
+In case error is reported, you can get following information about the error:
+
+- **Response code**: Error response code which native Install Referrer Library might return. Full list of potential response codes can be found in [here](https://developer.android.com/reference/com/android/installreferrer/api/InstallReferrerClient.InstallReferrerResponse) (`OK` will never be reported in this property, since it's a success status code). **Note**: Error code field is not always present in error map - only if error created when one of the error codes from native Install Referrer Library is received; otherwise this field will be **undefined**.
+- **Message**: Additional string message which describes error more in detail. **Note**: Message field should always be present in error map.
+
+## Under the hood
+
+Important thing to notice is that in order to work properly, Play Install Referrer Library requires following permission to be added to your app's `AndroidManifest.xml`:
+
+```xml
+<uses-permission android:name="com.google.android.finsky.permission.BIND_GET_INSTALL_REFERRER_SERVICE"/>
+```
+
+Play Install Referrer Library is added to **react-native-play-install-referrer** plugin as an [Gradle dependency](./plugin/android/build.gradle#L30) and it will automatically make sure that manifest file ends up with above mentioned permission added to it upon building your app.