Spruce up haas.cfg.example.

There are now two example configs, one for development, another for
production. The comments in both have been adjusted to be more useful.
Update README, write HACKING.

The README is now less developer-biased; It points to the INSTALL and
HACKING docs as appropriate.

This patch also updates a few things that were out of date, e.g. the
direct use of IPMI by users, and the mac address parameter to the
headnode_create_hnic call in one of the examples.
Address @ryabouza's comments.
README: Fix broken link.

I'd apparently lost the footnote part of the footnote link, but @gsilvis and I
are in agreement that this was kindof gross anyway -- "click here" is not
something one should write in a document that may be read in source form.
I've reworded slightly instead.
Merge pull request #353 from zenhack/intro-docs-cleanup

Intro docs cleanup