Add changelog for 0.19.0-rc1

CHANGELOG.md

+## 0.19.0-rc1 (2024-05-15)
+
+### Added (1 change)
+
+- [Document best practice for lockfile handling](components/opentofu@e317401e6b3d8cb1b3b7f8f341eb6d3ab046961a) ([merge request](components/opentofu!64))
+
+### Fixed (1 change)
+
+- [Mock CI_SERVER_HOST predefined env variable for unit test](components/opentofu@1e5b1e3c269d4a38c986bdfec8dbd69456de79c2) ([merge request](components/opentofu!65))
+
 ## 0.18.0-rc5 (2024-04-08)
 
 No changes.