Add release checklist

CHECKLIST-Release.md

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+# Release checklist
+
+- Upgrade Docs prior to Release
+
+    * Change log
+    * New features documented
+    * Update the contributor list - thank you page
+
+- Upgrade and test Reference Deployments
+
+- Release software
+
+    * Make sure 0 issues in milestone
+    * Follow release process steps
+    * Send builds to PyPI (Warehouse) and Conda Forge
+
+- Blog post and/or release notebook
+
+- Notify users of release
+
+    * Email Jupyter and Jupyter In Education mailing lists
+    * Tweet (optional)
+
+- Increment the version number for the next release
+
+- Update roadmap