CHANGELOG.md: Prepare v0.18.0 (prometheus#80)

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [0.18.0] - unreleased
+## [0.18.0]
 
 ### Changed
 
@@ -16,6 +16,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   replacing `owning_ref` with a similar crate, we switch to locking via
   `parking_lot` which supports the above mapping natively.
 
+  See [PR 78] and [issue 77].
+
+[PR 78]: https://github.com/prometheus/client_rust/pull/78
+[issue 77]: https://github.com/prometheus/client_rust/issues/77
+
 ## [0.17.0]
 
 ### Changed