DOC: Link to Python standard encodings from read_csv documentation

Trying to make it easier to access the list of encodings from the documentation. Adding this was suggested on this [SO question](http://stackoverflow.com/questions/25530142/list-of-pandas-read-csv-encoding-list/25530279#25530279).
I would just include only one link (not seperately for python 2/3), as the list is mostly the same in any case