Add a compatibility note for macOS users.
## 2.29.0-rc.0 / 2021-07-29
+Note for macOS users: Due to [changes in the upcoming Go 1.17](https://tip.golang.org/doc/go1.17#darwin),
+this is the last Prometheus release that supports macOS 10.12 Sierra.
+
* [CHANGE] Promote `--storage.tsdb.allow-overlapping-blocks` flag to stable. #9117
* [CHANGE] Promote `--storage.tsdb.retention.size` flag to stable. #9004
* [FEATURE] Add Kuma service discovery. #8844
