docs: add changelog for workaround migration

Author: anvanvan

CHANGELOG.md

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+# Changelog
+
+## Unreleased (2025-01-13)
+
+### Added
+- `canEdit()` convenience method to GetCalendarResponse for checking calendar write permissions (commit f1c2c81)
+  - Returns true for writable calendars or when privilege-set not provided by server
+  - Eliminates need for null-checking `isWritable()` in application code
+- `getCredentials()` method to CalDavClient for accessing stored credentials (commit 126d2fb)
+  - Returns array with 'user' and 'password' keys
+  - Removes need for reflection to access private properties
+- `parseVAlarms()` static method to GetCalendarResponse with full RFC 5545 support (commit fa44101)
+  - Supports duration-based triggers: "-PT15M", "PT0S", "-P1D", "-PT1H30M"
+  - Supports absolute DATE-TIME triggers: "19760401T005545Z"
+  - Handles mixed time units (weeks, days, hours, minutes, seconds)
+  - Preserves X-APPLE-DEFAULT-ALARM property for default alarm detection
+- `expandWithRRulePreservation()` static method to GetCalendarResponse (commit 8022062)
+  - Preserves RRULE on expanded recurring event instances
+  - Attaches RRULE as X-MASTER-RRULE property for frontend display
+  - Eliminates need to manually capture/restore RRULE during expansion
+- `createEventFromICS()` and `updateEventFromICS()` methods to CalDavClient (commit d66c0c7)
+  - Support for raw iCalendar content in PUT requests
+  - Enables proper all-day event creation and custom property preservation
+  - Returns EventCreatedResponse/EventUpdatedResponse with ETag handling
+- Comprehensive test coverage (commit 711ec62)
+  - Verified response methods (isSuccessFull, getCode) exist
+  - Tests for all new functionality
+
+### Improved
+- `getCurrentUserPrivileges()` and `isWritable()` methods now in GetCalendarResponse
+- All-day event creation/update now supported via library methods
+- VALARM parsing supports all RFC 5545 duration formats and absolute timestamps
+
+### Migration Notes
+For applications migrating from workarounds to native fork functionality:
+
+1. **Privilege Checking**: Replace null-checking logic with simple `$response->canEdit()` call
+2. **Credentials Access**: Replace reflection code with `$client->getCredentials()`
+3. **VALARM Parsing**: Replace manual parsing with `GetCalendarResponse::parseVAlarms($vevent)`
+4. **RRULE Preservation**: Replace manual RRULE capture/restore with `GetCalendarResponse::expandWithRRulePreservation($vcalendar, $start, $end)`
+5. **Event Creation**: Replace direct HTTP PUT/Guzzle calls with `$client->createEventFromICS()` and `$client->updateEventFromICS()`
+
+All changes are backward compatible with existing CalDavClient API usage.