Update CHANGELOG.md

Author: stepancheg

CHANGELOG.md

@@ -33,6 +33,12 @@ Version 3.0 is backward incompatible with 2.0 version. Changes are listed here:
 - `generate_getter` option to disable generation of getters functions.
 - [Flush `CodedOutputStream` on `drop`](https://github.com/stepancheg/rust-protobuf/commit/0e9cc5964c2731a771725bcf70125d3eb1c273b3)
 
+## [2.10] - Unreleased
+
+### Backward compatibility
+
+- Minimum supported Rust version is 1.40.0 now
+
 ## [2.9] - Unreleased
 
 ## [2.9.0] - (2019-09-30) yanked