[Tails-dev] Feature #5641 Explain why tor is slow

Hi,

a draft for feature 5641 has been done by Vidgis, which I added in git
in order to make it easier to review/merge etc.

I wrote a litlle introduction to Vidgis' draft and almost randomly
decided to place that file in /doc/advanced_topics/.

Please review that work here:

commit b54502c, doc/explain_why_Tor_is_slow branch, mercedes508
repo.

Cheers.