Add migration notice for new release (#740)

migration-notice.md

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+# Migration notice
+
+## CodePush SDK
+
+CodePush SDK migrated to a new service. We recommend updating CodePush SDK to the latest version. Versions lower than **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)** will not work in the near future.
+
+### Deprecated methods
+
+* getSessions
+* patchAccessKey
+* removeSession
+* getAccessKey
+
+These methods are not supported in versions **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)** and above.
+
+### Comparison
+
+* Server URL
+
+  * versions **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)** and above:
+
+    `https://api.appcenter.ms/v0.1`
+
+  * versions lower than **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)**:
+
+    `https://codepush-management.azurewebsites.net`
+
+    `https://codepush.appcenter.ms/v0.1/legacy`
+
+* Request's path
+
+    For example the path of `getDeployments` method:
+
+  * versions **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)** and above:
+
+    `https://api.appcenter.ms/v0.1/apps/{owner_name}/{app_name}/deployments/`
+
+  * versions lower than **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)**:
+
+    `https://codepush-management.azurewebsites.net/apps/{app_name}/deployments/`
+
+* Error message
+
+  Messages and status codes for error handling are different. For example the output of the case when you don't have the app:
+
+  * versions **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)** and above:
+
+    ```javascript
+    {
+        message: 'Not found. Correlation ID: *****',
+        statusCode: 404
+    }
+    ```
+
+  * versions lower than **[4.0.0](https://github.com/microsoft/code-push/releases/tag/v4.0.0)**:
+
+    ```javascript
+    {
+        message: 'Internal Server Error',
+        statusCode: 500
+    }
+    ```
+
+* Method signature
+
+    Methods signature was not changed. All methods work the same as in previous versions.
+
+## CodePush CLI
+
+The CodePush CLI **[3.0.0](https://www.npmjs.com/package/code-push-cli/v/3.0.0)** is the latest and last version for this CLI. We no longer update the CodePush CLI and recommend migrating to the App Center CLI (<https://github.com/microsoft/appcenter-cli>).
+
+## Cordova-plugin-code-push
+
+Cordova-plugin-code-push versions lower than **[1.12.0](https://github.com/microsoft/cordova-plugin-code-push/releases/tag/v1.12.0)** will stop working in the near future.
+
+## React-native-code-push
+
+React-native-code-push versions lower than **[5.7.0](https://github.com/microsoft/react-native-code-push/releases/tag/v5.7.0)** will stop working in the near future.