Update CHANGELOG.md for view loading time API

[ganeshnj]

What and why?

Changelog wasn't super clear.

How?

• make it clear that API is experimental
• make it clear the new required param is breaking change

Review checklist

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration)
• [ ] Make sure each commit and the PR mention the Issue number or JIRA reference
• [ ] Add CHANGELOG entry for user facing changes

[datadog-datadog-prod-us1[bot]]

Datadog Report

Branch report: ganeshnj/fix/update-view-loading-time-changelog Commit report: 1ce2dfc Test service: dd-sdk-ios

:white_check_mark: 0 Failed, 3468 Passed, 0 Skipped, 2m 24.04s Total Time :small_red_triangle_down: Test Sessions change in coverage: 4 decreased, 1 increased (+0.06%), 9 no change

:small_red_triangle_down: Code Coverage Decreases vs Default Branch (4)

• test DatadogCoreTests tvOS 78.67% (-0.04%) - Details
• test DatadogCrashReportingTests tvOS 26.8% (-0.02%) - Details
• test DatadogCoreTests iOS 72.69% (-0.01%) - Details
• test DatadogSessionReplayTests iOS 30.63% (-0.01%) - Details