Update README.md

Add example for appending an extension to files that do not have an extension

src/modules/powerrename/README.md

@@ -76,6 +76,7 @@ Matching and variable examples:
 | (.\*).png  | foo\_$1.png   | Prepends "foo\_" to the existing file name |
 | (.\*).png  | $1\_foo.png   | Appends "\_foo" to the existing file name  |
 | (.\*)      | $1.txt        | Appends ".txt" extension to existing file name |
+| (^\w+\.$)\|(^\w+$) | $2.txt | Appends ".txt" extension to existing file name only if it does not have an extension |
 
 
 ### External Help