prepare changelog (#717)

Prepare Changelog for Automation (#717)

This PR prepares the changelog to be automatically updated during releases. The contents of the pre-release body linked in this PR will be copied into CHANGELOG.md at release time.

Authors:
  - AJ Schmidt (@ajschmidt8)

Approvers:
  - Dillon Cullinan (@dillon-cullinan)

URL: #717

CHANGELOG.md

@@ -1,10 +1,6 @@
 # RMM 0.19.0 (Date TBD)
 
-## New Features
-
-## Improvements
-
-## Bug Fixes
+Please see https://github.com/rapidsai/rmm/releases/tag/branch-0.19-latest for the latest changes to this development branch.
 
 # RMM 0.18.0 (24 Feb 2021)