[PATCH] unshare system call -v5: Documentation file

Documents the new feature, why it is needed, it's cost, design,
implementation, and test plan.
Signed-off-by: Janak Desai <janak@us.ibm.com>
Cc: Al Viro <viro@ftp.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andi Kleen <ak@muc.de>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Michael Kerrisk <mtk-manpages@gmx.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>