Prepare Changelog for Automation (rapidsai#688)

This PR prepares the changelog to be automatically updated during releases.

Authors:
  - AJ Schmidt (@ajschmidt8)

Approvers:
  - Mark Harris (@harrism)
  - Keith Kraus (@kkraus14)

URL: rapidsai#688

Author: ajschmidt8

CHANGELOG.md

@@ -1,15 +1,6 @@
-# RMM 0.18.0 (Date TBD)
-
-## New Features
-
-- PR #636 Add a `Stream` class to manage CUDA streams in Python
-
-## Improvements
-
-- PR #647 Simplify `rmm::exec_policy` and refactor Thrust support
-
-## Bug Fixes
+# 0.18.0
 
+Please see https://github.com/rapidsai/rmm/releases/tag/branch-0.18-latest for the latest changes to this development branch.
 
 # RMM 0.17.0 (10 Dec 2020)