[RFC][DRAFT] Updated guide for upgrading to rich-roots [and the 2.0 beta formats]

[Alexander Belchenko]

I've just tested upgrade of qbzr trunk (revno.769) to 2a format.
Everything seems went OK, repo shrinks 4.5M -> 1.5M.
But after upgrade I have 4 or 5 packs in the repo, and only when I've 
repacked it I can see major speed-up in operations.
IIRC, this problem about needing pack after upgrade is known,
but I'm mentioning it anyway.

IIUC, 2a is not the latest one bbc format?
I will wait for BIG announcement when bzr will be ready
for final upgrade.

Thanks.