Direct new contributors to the eventloop-2.0 branch (#903)

Author: aloucks

README.md

@@ -10,6 +10,11 @@
 winit = "0.19.1"
 ```
 
+# Please make PRs against `eventloop-2.0` branch.
+
+All development work for our next version is being done against the [eventloop-2.0](https://github.com/rust-windowing/winit/tree/eventloop-2.0) branch. 
+Refer to [#459](https://github.com/rust-windowing/winit/issues/459) and [the associated milestone](https://github.com/rust-windowing/winit/milestone/2) for details on what that branch changes.
+
 ## [Documentation](https://docs.rs/winit)
 
 For features _within_ the scope of winit, see [FEATURES.md](FEATURES.md).