Chapter "Introduction" (introduction)

I noticed a few broken links:

* http://{www.,}erlang-projects.org/ appears to be down
* the Sun Microsystems paper moved to a different location (now https://blogs.oracle.com/kevinr/resource/Moodle-Sun-RA.pdf)
* the link to `CONTRIBUTORS` on https://git.process-one.net appears to be dead. I added a link to the GitHub.

http://www.vthd.org/ seems to be dead too, but I didn't want to just remove it. Any ideas on that link?