doc: fix arc42 link
We use decision template is based on
[Using patterns to capture architectural decisions](http://www.cs.rug.nl/~paris/papers/IEEESW07.pdf)
-and [arc42 decisions](http://confluence.arc42.org/display/templateEN/9.+Design+Decisions)
+and [arc42 decisions](http://docs.arc42.org/section-9/)
To add a new decision copy template.md and add a link in the appropriate
section here.
