rename.md: add -f flag (#960)

* rename.md: add note about -f flag

* rename.md: add -f example

pages/common/rename.md

@@ -10,7 +10,11 @@
 
 `rename -n {{'s/foo/bar/'}} {{\*}}`
 
-- Convert filenames to lower case:
+- Force renaming even if the operation would overwrite existing files:
+
+`rename -f {{'s/foo/bar/'}} {{\*}}`
+
+- Convert filenames to lower case (use `-f` in case-insensitive filesystems to prevent "already exists" errors):
 
 `rename 'y/A-Z/a-z/' {{\*}}`