[MLton] Updated http://mlton.org/RunningOnFreeBSD with a gotcha

[Matthew Fluet]

> I found a gotcha on FreeBSD. By default on FreeBSD versions 5.x, 6.x
> and 7.xa single
> process can never have more than 512M of memory unless you tune kernel
> parameters.
> I've documented it on the page http://mlton.org/RunningOnFreeBSD.

Thanks, that's very helpful.  BTW, is there a way to adjust the 
parameters in a per-program manner?

> While here, I propose the change of the FreeBSD default install prefix to
> /usr/local
> 
> in order to make it more on par with FreeBSDs hier(7) man-page.

That's useful as well.  Thanks for the commit.