doc: coal add warning about VM memory

[unclejack]

This warning would be useful when setting up CoaL the first time, especially when the disk is thrashing.

[dwlf]

Would it be sufficient to bold the step text for memory?

"Processes & Memory > set memory to 8192 MB or greater. Be sure to leave Mac OS X with at least 8 GB."
https://github.com/joyent/sdc/blame/master/docs/developer-guide/coal-setup.md#L99

[unclejack]

@lloydde Fair enough, I've followed the steps from the README the first time and didn't see that part.

I'll update the PR.

[rmustacc]

In this case was it coal on a Mac backed by an ssd or by a traditional spinning disk? In the former, I run quite comfortably in 6000 MB.

[unclejack]

@lloydde I've made the change. PTAL

@rmustacc It was a quick test on a spinning disk. I know not to do that again in the future.

Perhaps it would be worth mentioning that a regular spinning disk would benefit from more RAM or maybe just consider using an SSD right away?

[dwlf]

@unclejack oh ok, it doesn't look like the readme includes updating the memory.
https://github.com/joyent/sdc#cloud-on-a-laptop-coal

@rmustacc good to know. I'll have to 6000 MB a try.

I did try pursue getting "Coal headnode should default to 8GB RAM" [1], but maybe that was shot down because there is a lower reasonable number for general development and test as @rmustacc suggests.
[1] https://smartos.org/bugview/HEAD-2216