docs: Changelog update for 0.15.1 release.

[alesstimec]

Description

Adds the changelog entry for the 0.15.1 release.

Fixes:

Type of change

<What type of a change is this? Please keep one or more relevant lines from below and delete the others.>

• Add new resource
• Change existing resource
• Logic changes in resources (the API interaction with Juju has been changed)
• Change in tests (one or several tests have been changed)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Requires a documentation update
• Maintenance work (repository related, like Github actions, or revving the Go version, etc.)
• Other (please describe)

Environment

• Juju controller version:

• Terraform version:

QA steps

Manual QA steps should be done to test this PR.

provider juju {}
...

Additional notes

<Please add relevant notes & information, links to mattermost chats, other related issues/PRs, anything to help understand and QA the PR.>

[anvial]

Just add the desc. Thx!