add data for Nate's methodology feature

forecast-methodology/README.md

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+### Airline safety data
+
+The data behind the story, [Here’s What’s Under The Hood Of Our Senate Forecasting Model](http://fivethirtyeight.com/features/heres-whats-under-the-hood-of-our-senate-forecasting-model)
+
+Header | Definition
+---|---------
+`state` | Election
+`year` | Year of election
+`candidate` | Last name
+`forecast_prob` | Probability of winning election per FiveThirtyEight Election Day forecast
+`result` | `Win` or `Loss`
+
+For archived results, see:
+
+ * *2008* https://web.archive.org/web/20081106113055/http://www.fivethirtyeight.com/
+ * *2010* http://elections.nytimes.com/2010/forecasts/senate
+ * *2012* http://fivethirtyeight.blogs.nytimes.com/fivethirtyeights-2012-forecast/