Add a FAQ answer about why jemalloc is the default allocator

[alilleybrinker]

Per solson's comment on the latest round of new FAQ answers, there should probably be an explanation about why jemalloc is the default allocator for Rust programs. I'm not familiar with the reasoning behind the choice, but if someone else can give a solid explanation I'm happy to write it up and make the PR.

[alilleybrinker]

I've actually gotten a good answer about it from the rust-internals IRC channel, so I'll go ahead and write it up in the current PR.