Add release notes for 0.15.2 (openmc-dev#3357)

Release notes for 0.15.1 (openmc-dev#3340)

Co-authored-by: Patrick Shriwise <[email protected]>

Release of version 0.15.0 (openmc-dev#3050)

Release of version 0.14.0 (openmc-dev#2754)

Merge pull request openmc-dev#2441 from paulromano/release-0.13.3

Release of version 0.13.3

Merge pull request openmc-dev#2269 from paulromano/release-0.13.2

Release of version 0.13.2

Merge pull request openmc-dev#2182 from paulromano/release-0.13.1

Final change for release of version 0.13.1

Merge pull request openmc-dev#1967 from paulromano/release-0.13.0

Final change for version 0.13.0

Merge pull request openmc-dev#1848 from paulromano/hotfix-release-0.12.2

Hotfix release 0.12.2

Merge pull request openmc-dev#1803 from paulromano/fix-releasenotes

Fix release notes for 0.12.1