beta OPAM instructions

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

Thomas and I have been slogging away at getting the shiny new OCaml package 
manager working.  It's a little more complex than your average package manager 
due to the very strict static typing rules for OCaml libraries (including a CRC 
check to make sure you dont get them wrong).

Anyhows, the disaggregation works well enough that the Mirage website can be 
recompiled using OPAM (both Xen and UNIX), and we have a nice set of separate 
repositories to work from.  A *huge* upside here is that it's easier to use 
external OCaml libraries.

However, I could use some wider testing before I update the website.  If anyone 
could follow:

https://raw.github.com/mirage/mirage-www/master/tmpl/wiki/install-opam.md

On both Linux and MacOS X and get back to me, that would be most useful.  Once 
we're confident it works without catastrophe, I'll update the website.

Next steps (much quicker) are to activate the remaining libraries 
(OpenFlow/DNS/Block) and tag the developer preview release with more 
documentation.  Getting there!

-anil