sysfs-namespaces: add a high-level Documentation file

The first three paragraphs are almost verbatim taken from Eric's
commit message on the patch introducing network ns tags.  The next
two paragraphs I wrote to be a brief high level overview.  The last
section is taken from the commit message on "Implement sysfs tagged
directory support", but updated.  Hopefully correctly.

Signed-off-by: Serge E. Hallyn <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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