Update README

Author: xD8A

README.rst

@@ -213,6 +213,10 @@ To use the extensions below you must import from `jsonpath_ng.ext`.
 +==============+===============================================+
 | len          | - ``$.objects.`len```                         |
 +--------------+-----------------------------------------------+
+| keys         | - ``$.objects.`keys```                        |
++--------------+-----------------------------------------------+
+| str          | - ``$.field.`str()```                         |
++--------------+-----------------------------------------------+
 | sub          | - ``$.field.`sub(/foo\\\\+(.*)/, \\\\1)```    |
 |              | - ``$.field.`sub(/regex/, replacement)```     |
 +--------------+-----------------------------------------------+