Add note about testflight link

Sick of getting asked about this.

README.md

@@ -34,6 +34,8 @@ If you are looking to install or test osu! without setting up a development envi
 | [Windows (x64)](https://github.com/ppy/osu/releases/latest/download/install.exe)  | [macOS 10.12+](https://github.com/ppy/osu/releases/latest/download/osu.app.zip) | [Linux (x64)](https://github.com/ppy/osu/releases/latest/download/osu.AppImage) | [iOS(iOS 10+)](https://osu.ppy.sh/home/testflight) | [Android (5+)](https://github.com/ppy/osu/releases/latest/download/sh.ppy.osulazer.apk)
 | ------------- | ------------- | ------------- | ------------- | ------------- |
 
+- The iOS testflight link may fill up (Apple has a hard limit of 10,000 users). We reset it occasionally when this happens. Please do not ask about this. Check back regularly for link resets or follow [peppy](https://twitter.com/ppy) on twitter for announcements of link resets.
+
 - When running on Windows 7 or 8.1, **[additional prerequisites](https://docs.microsoft.com/en-us/dotnet/core/install/dependencies?tabs=netcore31&pivots=os-windows)** may be required to correctly run .NET Core applications if your operating system is not up-to-date with the latest service packs.
 
 If your platform is not listed above, there is still a chance you can manually build it by following the instructions below.