The namespaces compatibility list doc

People discuss how the namespaces are working/going-to-work together.

Ted Ts'o proposed to create some document that describes what problems user
may have when he/she creates some new namespace, but keeps others shared.
I liked this idea, so here's the initial version of such a document with
the problems I currently have in mind and can describe somewhat audibly -
the "namespaces compatibility list".

The Documentation/namespaces/ directory is about to contain more docs about
the namespaces stuff.

Thanks to Cedirc for notes and spell checks on the doc, to Daniel for
additional info about IPC and User namespaces interaction and to Randy, who
alluded me to using a spell checker before sending the documentation :)

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Daniel Lezcano <dlezcano@fr.ibm.com>
Cc: Theodore Tso <tytso@mit.edu>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>