[jit] Add Sphinx to install.texi

[Gerald Pfeifer]

On Monday 2014-10-20 15:19, David Malcolm wrote:
> +@item Sphinx (any working version)

As opposed to "any non-working version"? ;-)  I'd just omit 
"working" from this.

Otherwise this looks good to me, except...

> +Necessary to regenerate @file{jit/docs/_build/texinfo} from the .rst
> +files in the directories below @file{jit/docs}.

...that it probably should read @file{.rst}?

Gerald