editors/emacs-devel: Note that NATIVECOMP is experimental

Building Emacs with support for compiling elisp to native code is an
interesting idea, but up to now there are no obvious performance
improvements. There are problems though. Background compilation can
eat up CPU, Emacs may fail to build (e.g., bug 2629490), and some
dependent ports may encounter problems. Thus, note that the option is
experimental to give users reasonable expectations.

Discussed with: abbe, yasu
Sponsored by: The FreeBSD Foundation