Update Readme.md

Updated Readme.md to include information about some extra steps to support for the desktop - macOS platform to connect to the Web and retrive/store information on Parse server due to the changes in the macOS framwork after macOS 10.7 so that user's using the Parse SDK Flutter can get their app running quick and without issues.

Author: ManasMalla

packages/flutter/README.md

@@ -52,6 +52,19 @@ Due to Cross-origin resource sharing (CORS) restrictions, this requires adding `
 When running directly via docker, set the env var `PARSE_SERVER_ALLOW_HEADERS=X-Parse-Installation-Id`.
 When running via express, set [ParseServerOptions](https://parseplatform.org/parse-server/api/master/ParseServerOptions.html) `allowHeaders: ['X-Parse-Installation-Id']`.
 
+#### Desktop Support (macOS)
+Due to security entitlements posed by the macOS framework, connecting to the Web and sharing data requires adding the following lines to code :
+```
+<key>com.apple.security.network.client</key>
+<true/>
+```
+to the following files:
+```
+/macOS/Runner/Release.entitlements
+/macOS/Runner/DebugProfile.entitlements
+```
+to help the Parse SDK for Flutter communicate with the Web to access the server and send/retrive data.
+
 #### Network client
 By default, this SDK uses the `ParseHTTPClient`.
 Another option is use `ParseDioClient`. This client supports the most features (for example a progress callback at the file upload), but a benchmark has shown, that dio is slower than http on web.