Document merge command in README

Add documentation for the libby merge command that creates M4B audiobooks from individual chapter MP3 files. Includes usage examples and details about chapter markers, metadata embedding, and audio encoding settings.

Author: pdugan20

README.md

@@ -17,6 +17,7 @@ Download audiobooks from Libby to your computer for offline listening.
 
 - **One-Click Downloads**: Download audiobooks with a single click from your browser
 - **Interactive CLI**: Optionally add book title, author, narrator, and cover art to MP3 files
+- **M4B Audiobook Creation**: Merge individual chapter MP3s into a single M4B audiobook file with chapter markers
 
 ## Quick Start
 
@@ -72,6 +73,7 @@ libby
 Options:
 
 - Tag MP3 files (add metadata)
+- Merge chapters into M4B audiobook
 - List all downloaded books
 - View book details
 
@@ -90,6 +92,25 @@ libby tag ~/path/to/folder/ --title "Title" --author "Author"
 
 Embedded metadata: title, author, narrator, track number, cover art, description
 
+### Merge Command
+
+```bash
+# Interactive (shows list of books)
+libby merge
+
+# Merge specific folder
+libby merge ~/Downloads/libby-downloads/BookTitle/
+```
+
+Merges individual chapter MP3 files into a single M4B audiobook with:
+
+- Chapter markers (one per MP3 file)
+- Embedded metadata (title, author, narrator, cover art)
+- 64kbps AAC mono audio (optimized for voice)
+- M4B output file saved in the same directory
+
+Requires FFmpeg (bundled automatically).
+
 ### List Command
 
 ```bash