Update FAQ to add entry about long-term performance

As you mentioned, here's a pull request to add a FAQ entry about long-run performance.  I wasn't sure what you would want to say about JVM_OPTS in general, so I stuck to this simpler question for now.  If you don't think this is helpful and would like me to draft something about the general state of affairs with :jvm-opts and startup time instead, let me know.  

I also rearranged the Faster wiki page to make it clearer (in my option) -- hope this is OK.

https://github.com/technomancy/leiningen/wiki/Faster/_compare/5be4541a44146279e26ab4978843dea827fb036e...4fcec5ded3f44262e991f3110ea5a9a0139f11da

(Previously it jumped from 2.1 to enabling tiered compilation in 2.0 back to disabling it in 2.1 without being explicit about this -- I moved the part about disabling in 2.1 up next to the description of tiered compilation in 2.1).