Update CHANGELOG for upcoming release

No release yet, but wanted to write these things down

CHANGELOG.md

@@ -2,6 +2,38 @@
 
 --------------------------------------------------------------------------------
 
+## 0.2.51
+
+Not released yet
+
+### Added
+
+* The `wasm-bindgen-futures` and `wasm-bindgen-test` crates now require Nightly
+  Rust and have a new major version published as a result. These crates now
+  support `async`/`await` by default, and they will be supported in the stable
+  Rust 1.39.0 release. The previous versions of crates will continue to work on
+  stable today.
+  [#1741](https://github.com/rustwasm/wasm-bindgen/pull/1741)
+
+* Using `#[wasm_bindgen]` on an `async` function will now work and return a
+  `Promise` on the JS side of things.
+  [#1754](https://github.com/rustwasm/wasm-bindgen/pull/1754)
+
+* More helper methods for `js_sys::Array` have been added.
+  [#1749](https://github.com/rustwasm/wasm-bindgen/pull/1749)
+
+### Changed
+
+* The `passStringToWasm` function has been optimized for size.
+  [#1736](https://github.com/rustwasm/wasm-bindgen/pull/1736)
+
+### Fixed
+
+* BOM markers will not be preserved when passing strings to/from wasm.
+  [#1730](https://github.com/rustwasm/wasm-bindgen/pull/1730)
+
+--------------------------------------------------------------------------------
+
 ## 0.2.50
 
 Released 2019-08-19.