prepare changelog (#543)

Prepare Changelog for Automation (#543)

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: #543

CHANGELOG.md

@@ -1,3 +1,7 @@
+# dask-cuda 0.19.0 (Date TBD)
+
+Please see https://github.com/rapidsai/dask-cuda/releases/tag/branch-0.19-latest for the latest changes to this development branch.
+
 # dask-cuda 0.18.0 (24 Feb 2021)
 
 ## Breaking Changes 🚨