doc: Document dependencies required for building everything

Add a new section to README.DEVEL that describes the packages required
to build everything (including all optional code like the FUSE-based
user-mode client).  Start with what I figured out for FreeBSD (tested
on a clean 10.0 install) and what Russ Allbery described on the
openafs-devel list in
<https://lists.openafs.org/pipermail/openafs-devel/2014-February/019759.html>.

Change-Id: Ib90cd653a822f8699df613aabdd3442edc10c98a
Reviewed-on: http://gerrit.openafs.org/10844
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>