README.rst: add a note about the jsonpickleJS license

0f446ba267a81847108738cddadb006511d23037 is a git-subtree
merge of jsonpickleJS from https://github.com/cuthbertLab/jsonpickleJS.
This means we now include a reference javascript implementation.
Mention its license in our README as well.

Closes #76

Signed-off-by: David Aguilar <davvid@gmail.com>