Add changelog entry

Recently we found and fixed an unsoundness issue in the
`preallocated_gen_new` function. As we have yet to release 0.25.0 we can
just update the changelog to reflect the newly merged fix.

Author: tcharding

CHANGELOG.md

@@ -1,5 +1,7 @@
-# 0.25.0 - 2022-11-22
+    
+# 0.25.0 - 2022-12-07
 
+* [Fix soundness issue with `preallocated_gen_new`](https://github.com/rust-bitcoin/rust-secp256k1/pull/548)
 * Use type system to [improve safety](https://github.com/rust-bitcoin/rust-secp256k1/pull/483).
 * [Change secp256k1-sys symbol names to 0_6_1](https://github.com/rust-bitcoin/rust-secp256k1/pull/490).
 * [Introduce `rustfmt`](https://github.com/rust-bitcoin/rust-secp256k1/pull/499) to the codebase.