Update changelog (#40)

This PR updates the changelog for `cucim` to include some text that's necessary for our automated changelog scripts to function correctly.

Authors:
  - AJ Schmidt (https://github.com/ajschmidt8)

Approvers:
  - https://github.com/jakirkham

URL: #40

CHANGELOG.md

@@ -1,3 +1,7 @@
+# cuCIM 21.06.00 (Date TBD)
+
+Please see https://github.com/rapidsai/cucim/releases/tag/v21.06.00a for the latest changes to this development branch.
+
 # cuCIM 0.19.0 (15 Apr 2021)
 
 Initial release of cuCIM including cuClaraImage and [cupyimg](https://github.com/mritools/cupyimg).