Add Pin::as_deref_mut to relnotes

Author: coolreader18

RELEASES.md

@@ -60,6 +60,7 @@ Stabilized APIs
 - [`core::ptr::without_provenance_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.without_provenance_mut.html)
 - [`core::ptr::dangling`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling.html)
 - [`core::ptr::dangling_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling_mut.html)
+- [`Pin::as_deref_mut`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.as_deref_mut)
 
 These APIs are now stable in const contexts