Update README to require publisher name for ePub generation

README.md

@@ -5,20 +5,21 @@ GitHub action for converting markdown files to a combined ePub file.
 Sample of the action with the required inputs:
 ```yml
 - name: Create ePub
-  uses: harrymaynard/markdown-to-epub-action@v1
+  uses: harrymaynard/markdown-to-epub-action@v2
   with:
     markdownFiles: |-
       test-data/publication.md
       test-data/chapter-*.md
     title: My Book
     author: John Doe
+    publisher: Self-Published
 ```
 
 ## Inputs
 - `markdownFiles`: (required) Multi-line string containing markdown files to include. These can be indivitual files or globs.
 - `title`: (required) Title of the book.
 - `author`: (required) Author name.
-- `publisher`: (optional) Publisher name. If not specified, none is included in the ePub file.
+- `publisher`: (required) Publisher name.
 - `cover`: (optional) The book's cover image. This can be a URL (`https://example.com/cover.jpg`) or a path to the file located inside the repository (`images/cover.jpg`). If not specified, none is included in the ePub file.
 - `version`: (optional) Version of the ePub template to use. This can be either `3` or `2`. Default is `3` if not specified.
 - `lang`: (optional) Language code for the book. Default is `en` if not specified.