added rstanarm to web site

[Bob Carpenter]

I called it "RStanARM". If that's not right, let me
know.

Also, I can't get the fonts right in the bullets. I
hate markdown. As Michael diagnosed, if there's only
a single bullet point, everything gets scaled down in
font. You can see this in the interfaces, where RStanARM
and ShinyStan show up in single-item lists and are a bit
smaller. And also in the citations, though I hacked
a <big> tag in for McElreath, that actually overcompensates.

Anyone can edit this (it's on stan-dev repo stan-dev.github.io.

Michael provided a nice Wiki that worked for me to install the
Jekyll server:

https://github.com/stan-dev/stan-dev.github.io/wiki/Using-the-Jeykll-Based-Website

It's really cool the way it auto updates. But we need
a better theme.

I'm going to break citations down into subpages (like the interfaces)
and then add a bunch of case studies (then blog about it).

- Bob