docs(whiskers): document the --dry-run flag

whiskers/README.md

@@ -319,6 +319,11 @@ Whiskers can render multiple outputs from a single template using a matrix set
 in the frontmatter. This can be useful for generating one output per flavor per
 accent color, for example.
 
+In this mode Whiskers will render directly into a set of files as specified by
+the `filename` key in the frontmatter. This can be disabled with the `--dry-run`
+flag, in which case Whiskers will render the templates but not actually write
+them anywhere.
+
 The matrix is defined as a list of iterables. Whiskers will generate a file for
 each combination of the iterables in the matrix.
 
@@ -361,7 +366,9 @@ catppuccin-frappe-rosewater-normal.ini
 catppuccin-frappe-rosewater-no-italics.ini
 ```
 
-... and so on for every combination of flavor, accent, and variant.
+... and so on for every combination of flavor, accent, and variant. Notice that
+the filenames are generated by rendering the `filename` key in the frontmatter
+for each combination of the matrix iterables.
 
 ## Check Mode