add 'sub' line with function param names

Author: snopoke

README.rst

@@ -193,6 +193,7 @@ Extensions
 | len          | - ``$.objects.`len```                         |
 +--------------+-----------------------------------------------+
 | sub          | - ``$.field.`sub(/foo\\\\+(.*)/, \\\\1)```    |
+|              | - ``$.field.`sub(/regex/, replacement)```     |
 +--------------+-----------------------------------------------+
 | split        | - ``$.field.`split(+, 2, -1)```               |
 |              | - ``$.field.`split(sep, segement, maxsplit)```|