Patch to subversion documenation

This is designed to document how to set up read restrictions on portions
of the repository. It also documents what some of the problems and
caveats currently are. I'm not a documentation writer, so this could
perhaps be improved greatly. But, such a section in the documentation
would've saved me 4-5 hours of time spent fiddling around trying to
figure things out.

Have fun (if at all possible),

-- 
The best we can hope for concerning the people at large is that they
be properly armed.  -- Alexander Hamilton
-- Eric Hopper (hopper_at_omnifarious.org  http://www.omnifarious.org/~hopper) --