Add CHANGELOG.md, and append 0.4.0 release notes

Author: tankyleo

CHANGELOG.md

@@ -0,0 +1,13 @@
+# 0.4.0 - Nov 11, 2025
+- Enforce that all implementations of `VssHeaderProvider` are `Send+Sync` (#35).
+- Bump MSRV to 1.75.0 (#38).
+- Allow to set `aad` in `StorableBuilder::{build, deconstruct}` (#40).
+- Pass the `data_encryption_key` by reference to `StorableBuilder::{build, deconstruct}` (#40).
+- Replace LDK's `chacha20poly1305` implementation with the rust-bitcoin implementation (#42).
+- Rename `vss-client` crate to `vss-client-ng` (#46).
+- Enable `reqwest` client-level timeouts (#39).
+- Add `VssClient::from_client_and_headers` constructor (#39).
+
+In total, this release features 15 files changed, 205 insertions, 1162 deletions in 19 commits from 2 authors in alphabetical order:
+- Elias Rohrer
+- Leo Nash