Update arguments documentation

guide/arguments.md

@@ -164,6 +164,30 @@ Display some statistics (word and character counts) about the book.
 
 Equivalent to `--set proofread true`. Enable proofreading. See [Proofreading](proofreading.md).
 
+`--autograph`
+----------------
+
+**Usage**: `crowbook --autograph <BOOK>`
+
+(or `crowbook -a <BOOK>`)
+
+Prompts for a an autograph execution. This is a Markdown block that will be inserted at the
+beginning of the book.
+
+### Example ###
+
+```
+$ crowbook --autograph my.book
+CROWBOOK 0.14.0
+Enter autograph:
+To my dear friend John,
+
+Cheers, *Joan*
+^D
+```
+
+will add the block of text that was entered to all output files.
+
 `--verbose`
 -----------