set release date for 0.6.1
MODIFY
docs/development/release_notes.md
docs/development/release_notes.md
# Release Notes
-## v0.6.1 - Unreleased
+## v0.6.1 - 2021-10-25
- Major internal refactor of executors. {pull}`219`.
Began deprecation cycle for recipe methods (e.g. `recipe.prepare_target()`) in

set release date for 0.6.1