Document third configuration variable $PL_RPROMPT

[HardRockMartian]

Great project! I did discover that a third variable $PL_RPROMPT needs to be set if you want to control the powerline that appears on the far right after a command is entered. By default, this powerline shows the history number and the timestamp.

The README.rst mentions the other two prompt controlling variables $PL_PROMPT and $PL_TOOLBAR. Metioning $PL_RPROMPT would complete the trifecta.

Thanks again for providing this to the xonsh community!