add documentation for the diff-source configuration option

Author: doy

book/src/configuration.md

@@ -65,6 +65,7 @@ Its settings will be merged with the configuration directory `config.toml` and t
 | `workspace-lsp-roots` | Directories relative to the workspace root that are treated as LSP roots. Should only be set in `.helix/config.toml` | `[]` |
 | `default-line-ending` | The line ending to use for new documents. Can be `native`, `lf`, `crlf`, `ff`, `cr` or `nel`. `native` uses the platform's native line ending (`crlf` on Windows, otherwise `lf`). | `native` |
 | `insert-final-newline` | Whether to automatically insert a trailing line-ending on write if missing | `true` |
+| `diff-source` | The source to use for diff operations (gutter, `:diffg`, `]g`, etc). Can be `git`, `file` (uses the contents of the file on disk), or `none`. | `git` |
 
 ### `[editor.statusline]` Section