Add barebones GPUI README (#4235)

Marshall Bowers created

This PR adds a barebones README to the `gpui` crate, just so folks have
somewhere to land on when we link them here from
[gpui.rs](https://www.gpui.rs/).

We can flesh this out as we go.

Release Notes:

- N/A

Change summary

crates/gpui/README.md | 3 +++
1 file changed, 3 insertions(+)

Detailed changes

crates/gpui/README.md 🔗

@@ -0,0 +1,3 @@
+# GPUI
+
+A fast, productive UI framework from the creators of [https://zed.dev].