Add talks and articles to documentation

[tiran]

There have been a couple of tasks and articles about Custodia. Let's add a resource page to our documentation.

• https://ssimo.org/slides/Secrets.pdf (FLOCK 2015, recording available)
• https://speakerdeck.com/tiran/sharing-secrets-in-a-container-world (DevConf CZ 2017, recording available)
• https://speakerdeck.com/tiran/pycon-2016-file-descriptors-unix-sockets-and-other-posix-magic (PyCon US 2016 about Unix Socket auth, recording available)
• https://speakerdeck.com/tiran/sharing-secrets-in-a-container-world-confoo-montreal-2017 (ConFoo Montreal 2017)
• https://www.peerlyst.com/posts/sharing-secrets-with-containers-using-custodia-alan-robertson
• @frasertweedale is working on a talk
• IIRC @dpal has a presentation