Add wiki as a git submodule

[morancj]

The wiki for this repo was getting outdated: I recently removed reference to a Wordpress setup which went away in 2016 (!). Making the wiki more visible should help with that. Since submodules are already in use here, I have added another in preference to git subtree. :slightly_smiling_face:

Good summary of subtree usage: https://www.atlassian.com/git/tutorials/git-subtree