docs: add release notes for v2.2.1 (#9)

Author: mmonterroca

RELEASE_NOTES_v2.2.1.md

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+# Release Notes - v2.2.1
+
+**Release Date:** January 2026
+
+## Bug Fixes
+
+This release fixes critical DOCX validation errors that prevented documents from opening in Microsoft Word after round-trip operations (read → modify → save).
+
+### Fixed
+
+- **Hyperlink RelationshipID Preservation** ([#6](https://github.com/mmonterroca/docxgo/issues/6))
+  - When reading and re-saving documents with hyperlinks, the library was generating new relationship IDs instead of preserving the originals
+  - This caused Word to fail with "relationship not found" errors
+  - Now correctly preserves original `rId` references during round-trip
+
+- **Drawing Position Serialization**
+  - Fixed `wp:align` empty value error for floating images
+  - Images with `posOffset=0` were incorrectly serialized with empty `<wp:align>` elements
+  - Added `UseOffsetX`/`UseOffsetY` flags to `ImagePosition` to distinguish between "offset = 0" and "no offset set"
+
+- **Internal Hyperlink Anchors**
+  - Added support for anchor-based internal hyperlinks (bookmarks, TOC links)
+  - Serializer now correctly handles both `anchor` property and URLs starting with `#`
+
+- **Custom Styles Preservation**
+  - Documents with custom styles (modified Heading1, custom BodyCopy, etc.) are now preserved correctly during round-trip
+  - All 55+ custom styles from templates are maintained
+
+## New Features
+
+### Troubleshooting Documentation
+
+Added comprehensive troubleshooting guide for DOCX validation errors:
+- `docs/TROUBLESHOOTING_DOCX_VALIDATION.md`
+- Includes diagnostic workflow, common errors, and solutions
+
+## Installation
+
+```bash
+go get github.com/mmonterroca/docxgo/v2@v2.2.1
+```
+
+## Usage Example
+
+```go
+// Read existing document with custom styles
+doc, err := docx.OpenDocument("template.docx")
+if err != nil {
+    log.Fatal(err)
+}
+
+// Modify content - styles are preserved
+para := doc.AddParagraph()
+para.AddRun().SetText("New content")
+
+// Save - document opens correctly in Word
+if err := doc.SaveAs("output.docx"); err != nil {
+    log.Fatal(err)
+}
+```
+
+## Known Limitations
+
+When loading an existing document and adding **new** images or hyperlinks (not modifying existing ones), the new relationships may not be written correctly. This is tracked in [#8](https://github.com/mmonterroca/docxgo/issues/8).
+
+For the primary use case of reading a template, modifying text, and saving, this version works correctly.
+
+## Acknowledgements
+
+Thanks to [@krishnadubagunta](https://github.com/krishnadubagunta) for reporting the issue with detailed examples that helped identify the root cause.
+
+## Files Changed
+
+- `domain/image.go` - Added `UseOffsetX`/`UseOffsetY` flags
+- `internal/core/run.go` - Preserve existing relationshipID
+- `internal/core/document.go` - Store preserved parts for round-trip
+- `internal/reader/reconstruct.go` - Preserve original parts and set UseOffset flags
+- `internal/serializer/serializer.go` - Handle anchor hyperlinks, use preserved relationshipID
+- `internal/writer/zip.go` - Write preserved parts during round-trip
+- `internal/xml/drawing_helper.go` - Use offset flags in serialization
+- `internal/xml/paragraph.go` - Add Anchor/History to Hyperlink struct
+
+## Full Changelog
+
+See [PR #7](https://github.com/mmonterroca/docxgo/pull/7) for complete details.