Document kernel settings to run SheepShaver in Linux somewhere obvious

To run SheepShaver in Linux, one that was made with the default `configure` addressing setting, requires changing the kernel setting `vm.mmap_min_addr` to 0.

Otherwise you get an error like:
`$ SheepShaver`
`Cannot map Low Memory Globals: Operation not permitted.`

It would be nice if this was documented somewhere obvious like the readme.

You can set the setting by echoing 0 to `/proc/sys/vm/mmap_min_addr` as root, or doing

`sudo sysctl -w vm.mmap_min_addr=0`

and you can make it happen automatically at startup by putting

`vm.mmap_min_addr = 0`

into `/etc/sysctl.d/whatever.conf`

**Edit:**
The kernel null deref protection afforded by mmap minimum address enforcement has defense in depth value. If we can make some working build other than a real addressing build be the default `configure` option, great.

This issue is just a practical matter: while real addressing is still the default `configure` option, without instructions, users encounter the error message and show up in the issue tracker for support.