put changelog.md for release notes in repo (#2463)

Author: danoswaltCL

CHANGELOG.md

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+UpGrade 6.1 Release Notes
+
+Segments Refresh:
+
+- Redesigned UI for Segments pages / modals (UX design kudos to @zackcl)
+- Global exclusion segments now separated by app-context (was previously one global list)
+- Updated list management for segments with import/export functionality
+- Improved search and pagination
+
+Note that the segment data structure has been updated also:
+
+- Backwards compatible, old and new segment structures will both work.
+- Newly added segments will see an update UI for working with lists.
+- 'Old' segments will continue to be supported, but with the old UI in the details view.
+- It is recommended to recreate 'old' style lists to the newer style when convenient.
+
+Other updates:
+
+Metrics display in data tab of experiments has been turned back on in the UI
+Performance Optimizations - Improved database queries for metrics and experiment conditions
+Error Handling Improvements - Better HTTP error codes with consistent handling across endpoints
+Mooclet Integration Backend - Added foundational support for Mooclet experiments (not user-visible in this release)
+
+Angular 19 / Node 22 Support
+Java Client support for Java 17 (specifically handling PATCH requests)
+Support for ECS Infrastructure, CORS whitelisting
+
+Important note for devs running upgrade instance locally:
+- local UI configuration no longer uses the hardcoded `environment.ts` by default in local
+- you will need to create an `environment.local.ts` file, which is properly gitignored