Disable nightly CI and add README disclaimer

Bevy 0.6.0 is broken for nightly, which breaks all `docs.rs` builds.
Remove nightly CI and add a README warning until this is fixed in 0.6.1.

.github/workflows/ci.yaml

@@ -13,7 +13,7 @@ jobs:
   build:
     strategy:
       matrix:
-        toolchain: [stable, nightly]
+        toolchain: [stable]
         os: [windows-latest, ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:

README.md

@@ -8,6 +8,8 @@
 
 Tweening animation plugin for the Bevy game engine.
 
+⚠ _Bevy 0.6.0 does not build with the `nightly` toolchain. This breaks `docs.rs` builds. A fix is expected for 0.6.1._
+
 ## Features
 
 - [x] Animate any field of any component or asset, including custom ones.