Updated the broken link and contents of Contents.lr

This is pertaining to the issue #327 (closed)

I have updated the link to the blog https://blog.torproject.org/why-tor-slow-and-what-were-going-do-about-it/ , instead of https://blog.torproject.org/why-tor-is-slow , which was responsible for 404 page not found error

I also added new sentence which references to the ongoing DoS attacks on Tor: https://blog.torproject.org/tor-network-ddos-attack/ as suggested by @kez