In the Pulumi Cloud, there is no guarantee that two stacks will share
the same encryption key. This means that encrypted config can not be
shared across stacks (in the Pulumi.yaml) file. To mimic this behavior
in the local experience, we now use a unique key per stack.
When upgrading an existing project, for any stack with existing
secrets, we copy the existing key into this stack. Future stacks will
get thier own encryption key. This strikes a balance between
expediency of implementation, the end user UX and not having to make a
breaking change.
As part of this change, I have introduced a CHANGELOG.md file in the
root of the repository and added a small note about the change to it.
Fixes#769
