Add jekyll infrastructure

Author: hadley

.gitignore

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 *.pdf
 .Rproj.user
 .Rhistory
-cache
+_cache
+_site
+cache

CONTRIBUTING.md

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+The content of this wiki will be included in a forthcoming book from Chapman
+& Hall, and hence I need to own the copyright for the complete content
+of the wiki. I will make some money from the book, but it's not likely to be a 
+huge amount (probably <$10,000 over 2 years, which works out to a fairly low 
+hourly rate). The wiki will continue to be freely available and I'll acknowledge
+all contributions in the printed book.
+
+To confirm that you're ok with giving me your copyright, please include the 
+phrase: "I assign the copyright of this contribution to Hadley Wickham".
+
+If you have any questions, feel free to raise them in the pull request, or 
+send me [an email](mailto:[email protected]).