Make it clear that basically 0 active development is happening.

Mostly a documentation fix.

https://lists.torproject.org/pipermail/tbb-dev/2017-May/000556.html

Anyway, it's fairly clear to me that there is no clear plan for how to
handle sandboxing's requirements vs `tor-launcher`, so I will continue
with:

  "the only guaranteed maintenance it gets is that it will get fixes
   when it breaks on Yawning's laptop, or when people submit detailed
   bug reports that Yawning can fix in spare time"