Add detail about SLiM install to installation docs

[hyanwong]

It's unclear to me if I need to install SLiM separately from stdpopsim. My guess is that if I install via conda, it will install a working SLiM version for me, but if I install via pip, I will have to install SLiM separately. Is this right? Either way, it would be helpful to mention on the installation page if I need to do anything else to install the engines used (I assume msprime is always installed if necessary, FWIW)

[petrelharp]

Ah, thanks, @hyanwong - this is indeed something we need to sort out. I suspect you're right, but since stdpopsim is usable without SLiM I'm not even sure what's the right behaviour?

[hyanwong]

I think the current behaviour is fine as long as it's documented?