Update README

I think we have moved all documentation and we can remove this paragraph without any doubts. Also site with previous documentation is invalid now so it doesn't have any sense to me

README.md

@@ -1,9 +1,5 @@
 # 👋 Welcome
 
-{% hint style="warning" %}
-The migration of documentation is ongoing, and thus you may find some incomplete or missing docs. If in doubt, check out the [existing docs portal](http://avaloniaui.net/docs#).
-{% endhint %}
-
 Welcome to Avalonia Documentation, a steadily growing collection of guides, documentation and API reference for Avalonia UI. 
 
 If you can't find what you need here, [visit our friendly community ](https://gitter.im/AvaloniaUI/Avalonia)where you'll find lots of people happy to help you, or if you're in need of more support you can opt for a [paid support plan](https://avaloniaui.net/support.html) which helps support the continued growth of the project.