Improve the INSTALL doc.

> >>36631388
> I posted this the other day, but you didn't respond so I couldn't be sure you saw.
>
> Anyways, I'm the fellow that was installing guile-2d, and got a syntax
> error in mvars because of the docstring in the caselambda.
>
> I downloaded the fixed version of the file, and finally finished the
> installation.  Now I've just got to learn guile...
>
> While I was installing I wrote a few notes.
>
> 1) The tar zxf guile-sdl-<version> did not work, but tar xf
> guile-sdl-<version> did.
>
> 2) Guile isn't mentioned in the INSTALL.org. It was obvious this was
> my problem, and it just took a command to fix it, but it still
> would've been easier had you explicitly listed it.
>
> 3) https://gitorious.org/guile-figl/guile-figl needs ".git" added to
> the end of it.
>
> 4) libfreeimage3 and libftgl2 weren't enough. I ended up needing to
> install ftgl-dev and libfreeimage-dev. Not sure if both were needed,
> or just the dev versions.