Document that JIT should be avoided on tileservers

[hummeltech]

          We should document that JIT should be avoided on tileservers because of memory issues (and more general reasons) and close this issue and open a new issue for any new memory issues.

Originally posted by @pnorman in #181 (comment)

[pnorman]

As a follow-up to that comment I noted that the mod_tile related bugs bugs are closed so it's no longer an issue.

I still recommend turning off JIT, but that's because of stuff in the style, not mod_tile.