Adding MGRIT/XBraid mentions

Hi Daniel, Torbjörn,  

Jacob Schroder here.  We've finally gotten our project website up and running at 
http://www.llnl.gov/casc/xbraid
This is a pull request to put some brief mention of this website on parallelintime.org.  

It might be good to preview the new markdown pages, as I couldn't view things very well locally on my end.  But, I followed the instructions, so I think it's all good to go.

Hope all is well,

Jacob