added unstable-book entry for bindings_after_at

src/doc/unstable-book/src/language-features/bindings-after-at.md

@@ -0,0 +1,22 @@
+# `bindings_after_at`
+
+The tracking issue for this feature is [#65490]
+
+[#65490]: https://github.com/rust-lang/rust/issues/65490
+
+
+The `bindings_after_at` feature gate allows patterns of form `binding @ pat` to have bindings in `pat`.
+
+```rust
+#![feature(bindings_after_at)]
+
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+fn main() {
+    let point@ Point{x: px, y: py} = Point {x: 12, y: 34};
+}
+```
+