Some more ubifs docs

[Artem Bityutskiy]

Hi,

because many people are confused about where is UBIFS used,
I've added this piece of documentation:

http://www.linux-mtd.infradead.org/doc/ubifs.html#L_raw_vs_ftl

It also tries to answer the question like "what is better, FTL device +
traditional FS or raw flash + UBIFS". Because some things may appear to
be questionable, I'd ask people to take a look at this.

Thanks.

-- 
Best regards,
Artem Bityutskiy (Битюцкий Артём)