Removed not existing link from readme (#850)

There was no link for migration to v2 and changelog. It looks bad when dead links are on main readme, so I removed the link.

README.md

 * [Installation - iOS](https://github.com/wix/react-native-navigation/wiki/Installation---iOS#installation---ios)
 * [Installation - Android](https://github.com/wix/react-native-navigation/wiki/Installation---Android)
 * [Usage](https://github.com/wix/react-native-navigation/wiki/Usage)
-* [Migrating to version 2.0](https://github.com/wix/react-native-navigation/wiki/Migrating-to-version-2.0)
 * [Top Level API](https://github.com/wix/react-native-navigation/wiki/Top-Level-API)
 * [Screen API](https://github.com/wix/react-native-navigation/wiki/Screen-API)
 * [Styling the navigator](https://github.com/wix/react-native-navigation/wiki/Styling-the-navigator)
 * [Styling the tab bar](https://github.com/wix/react-native-navigation/wiki/Styling-the-tab-bar)
 * [Deep links](https://github.com/wix/react-native-navigation/wiki/Deep-links)
 * [Third party libraries support](https://github.com/wix/react-native-navigation/wiki/Third-party-libraries-support)
-* [Release Notes](https://github.com/wix/react-native-navigation/blob/master/CHANGELOG.md)
 * [Milestones](https://github.com/wix/react-native-navigation/wiki/Milestones)
 * [Contributing](https://github.com/wix/react-native-navigation/wiki/Contributing)
 * [License](https://github.com/wix/react-native-navigation/wiki#license)