Add info about the breaking changes wrt updating to 0.19 in the readme

Author: shana

README.md

@@ -1,11 +1,16 @@
 # [GitHub for Unity](https://unity.github.com)
 
+## Notices
+
+From version 0.19 onwards, the location of the plugin has moved to `Assets/GitHub`. If you have version 0.18 or lower, you need to delete the `Assets/Editor/GitHub` folder before you install newer versions. You should exit Unity and delete the folder from Explorer/Finder, as Unity will not unload native libraries while it's running.
+
 ![Build Status](https://ci.appveyor.com/api/projects/status/github/github-for-unity/Unity?branch=master&svg=true)
 
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+## About
 
 The GitHub for Unity extension brings [Git](https://git-scm.com/) and GitHub into [Unity](https://unity3d.com/), integrating source control into your work with friendly and accessible tools and workflows.